Prof. Michel Orrit has been awarded the NWO Spinoza Prize, the highest Dutch scientific award. He receives the award for his work in single molecule spectroscopy. Orrit receives 2.5 million euro to invest in his research.

because it allows us to look at differences between individual molecules', Oritt says at the Bessensap conference, where the winners were announced. 'Also the aspect of time is a factor that we can research with this technique.'

An ‘invisible giant’ in the world of spectroscopy is how NWO describes Professor Michel Orrit (1956). Colleagues from the Netherland and abroad praise him as one of the leading and most innovative researchers in single-molecule optics. This is the name given to his work when three physicists received a Nobel Prize in 2014 for the development of fluorescent microscopy. In spite of these eulogies, Orrit is a modest man, who puts all his energy into research, knowledge transfer and supervising young researchers. Nonetheless, the work of this level-headed Frenchman has not gone unnoticed by NWO: hence the award of the Spinoza Prize, the highest Dutch science award.

What did you think when you heard the news?
‘It came as a complete surprise. I thought I was too old for these kinds of awards! It goes without saying, the prize is not only an appreciation of my academic career to date, it is also a stimulus to continue and even expand the work over the work over the coming years. I may be slowly approaching the end of my career, but I can still carry out a lot of experiments with this 2.5 million euros. That’s exactly my definition of what science is all about: trying out things when you don’t know whether they’ll work or not. It’s important that there’s funding to do that.’

What is your particular expertise?
‘I study individual molecules. Around thirty years ago it wasn’t possible to study a single molecule. But at the end of the eighties, after a lot of trial and error, my American colleague William Moerner proved that it was possible. I did the same thing shortly afterwards, and I found even stronger evidence. I shone laser light on a molecule. The fluorescence – the light that the molecule reflects – was then ‘read’ with a small parabolic mirror that works more or less like a television satellite dish. We’ve improved this technique since then and, as well as fluorescence, we now use absorption to detect individual molecules. This gives us access to a much larger spectrum of molecules, because not all molecules fluoresce.’

What makes this research important for science and society?
‘Single-molecule spectroscopy makes it possible to show whether a particular protein is present in a cell or not. That’s an important step forward for medical research, for example. You can determine much faster whether someone is a carrier of a particular disease. Thanks to these scientific developments, it’s now much simpler and cheaper to map the genome of a living being. Whereas in the nineties it cost millions of dollars to work out a person’s genetic make-up, you can now do it in an afternoon and for less than a thousand euros. That’s really useful for forensic analyses. And the same technique was also used during a cholera outbreak in Haiti in 2010. Scientists were able to determine that Nepalese UN peace soldiers brought the disease to Haiti. But the applications outside the medical and forensic world are also enormous. We recently imaged conductive polymers for the first time ever. They’re used for making solar cells, and you need to know their characteristics in order to be able to make new solar cells work as efficiently as possible.’

What future research will you be able to do with this Spinoza Prize?

‘Right now, I’m doing research with colleagues on how gold nano-antennae work. These are a kind of mini grain of rice that is small enough to penetrate a cell membrane without causing irreparable damage to the cell. By attaching the nano-antenna to a single molecule in the cell, you can enhance the fluorescence of the molecule up to five hundred times. That way you can see the molecule better, but you can also influence it from a distance. You can make it rotate, for example, you can take hold of it or pull on it. Or you can heat it up and destroy it. We’re hoping we’ll be able to make a nanoscale scalpel that we can use to manipulate the molecules within a cell. I’m sure there will be all kinds of possible medical or chemical applications, but for the time being what’s important is to find out how it works.'

Universe or Multiverse?
Cosmological observations show that the universe is very uniform on the largest scales accessible to our telescopes, and the same laws of physics operate in all of its parts that we can see now. The best theoretical explanation More info

of this uniformity is provided by the inflationary theory, proposed 30 years ago. I will briefly describe the status of this theory in view of recent observational data obtained by the Planck satellite. Rather paradoxically, this theory predicts that on extremely large scales, much greater than what we can see now, the world may look totally different. Instead of being a single spherically symmetric balloon, our universe may look like a "multiverse," a collection of many different exponentially large balloons ("universes") with different laws of physics operating in each of them. In the beginning, this picture looked more like a piece of science fiction than a scientific theory. However, recent developments in inflationary cosmology, particle physics, and string theory provide strong evidence supporting this new cosmological paradigm. This is a profound challenge to the standard views on the origin and the global structure of the universe and on our own place in the world.

About the speaker
Andrei Linde (Stanford University) is one of the inventors of the theory of chaotic inflation, the most general form of inflationary cosmology, that provides our best current understanding of the origins and distribution of stars and galaxies in our universe. He is the author of the books Inflation and Quantum Cosmology and Particle Physics and Inflationary Cosmology. His honors include the Dirac Medal, Peter Gruber Prize, the Fundamental Physics Prize, and Kavli Prize. Together with Prof. Renata Kallosh he is a guest professor at the Institute Lorentz for Theoretical Physics for the summer of 2017.

Leiden University has appointed John van Noort as Professor in Biophysics. He studies the way in which our DNA is folded and read out.

John van Noort studied Molecular Life Sciences before he obtained his PhD in Applied Physics. The focus More info

of his PhD research and the subsequent postdoc has always been on the biological side of physics. After thirteen years as a group leader in Leiden, he is now offered a chair in Biophysics, which he will officially accept on February 16 during his oration. ‘I especially like the multidisciplinary character of this field: linking physics to biology and vice versa. It enables you solve new biological problems with quantitative techniques. Plus you get to work with interesting people from different backgrounds.’

Biology
The subject of Van Noort’s research gives it its biological aspect; organization and readout of DNA. He studies for example how hormone receptors bind to DNA and then influence a gene’s activity. An interesting factor here is how the DNA’s organization affects the binding of the hormone receptor. This way he learns more and more about the DNA’s readout process, that determines which of the proteins that the DNA encodes for actually are produced.

Physics
The microscopes that Van Noort and his group build give the research its physical aspect. ‘Knowledge on optics and mechanics is essential for that,’ he explains. ‘We also use lots of statistical physics in interpreting the data. We don’t just get to see full pictures, because we’re really working on the level of a single molecule. Those are too small to depict with optical microscopy. We can however analyze the emerging fluorescent signals, and find out the underlying structures.’

Medicine
At the start of his professorship, Van Noort expects to move closer towards pharmaceutical applications. Because he studies for example the binding of hormone receptors to DNA, his research could contribute to the development of hormonal drugs that influence this binding. Currently, these drugs often act as rough remedies. Cortison is for example prescribed for many different complaints, such as infections, pain or immune diseases, and has a long list of side effects. With new insights and screening techniques, pharmacists can develop more specific and therefore more effective hormonal drugs.

Professor Renata Kallosh (Stanford University), one of the world’s leading theoretical physicists, will be this summer’s Lorentz Professor at the Leiden institute for theoretical physics. Her main areas of interest are cosmology and string theory. She studied physics in Moscow, More info

where she also obtained her PhD in 1968. After being a Professor at the same institute, she moved to Switzerland in 1989 to work at CERN and a year later to California where she currently conducts her research at Stanford University. She will start her Lorentz Professorship in Leiden on June 12.

What do you expect from your Lorentz Professorship?
'I am very pleased with the Lorentz Professorship. To me this is one of the most valuable awards: Lorentz is my hero in physics. I hope to be able to present
lectures which will live up to expectations. This is a challenge. I also hope to meet many interesting people during my stay in Leiden and particularly during the workshop
which I hope will be very productive.'

About your work: you study the cosmological implications of string theory. Is it possible to explain this in words?
'I study the cosmological implications of string theory and supergravity. We are closing in on the first 10-35 seconds of the universe, using General Relativity and Quantum Field Theory. Supergravity and a more advanced version of it —superstring theory— are the only advanced forms known to us of General Relativity and Quantum Field Theory which we may test by cosmological observations in the early universe. And by the way, the fundamentals of supergravity were to a large extent developed in The Netherlands, by Bernard de Wit, Eric Bergshoeff and Mees de Roo, and in application to cosmology by a younger generation of people in The Netherlands like Ana Achucarro and Diederik Roest.'

To outsiders, string theory is famous for being a theory that is impossible to prove. Will string theory shrug off this image?
'I don’t have an answer to your question, but at present I see that string theory ideas help us to build cosmological models which fit the data from observations.
Moreover, we have produced relatively simple predictions from string theory and related theories which will be testable with future detectors of primordial gravity waves (gravitational waves from the Big Bang, edit.)'

What is the most promising upcoming observation regarding confirming your theoretical work?
'Future ground based or satellite missions will either detect primordial gravity waves or reduce the current bound on the ratio of tensor to scalar perturbations (amount of gravitational waves, edit.) during early universe inflation. Both of these outcomes will be of a significant importance for theoretical physics in general, and for my work in particular.'

As a student, what attracted you to cosmology in general and string theory in particular?
'I was working on supergravity and string theory during most of my career in physics since I was always interested in mathematical methods in physics. More recently I became involved into studies of cosmology and I found it gratifying to see that the results of my formal theoretical studies are known and discussed in the context of observational cosmology, for example in the Planck satellite comparison of theory with experiment and in upcoming experiments on the Cosmic Microwave Background.'

You will be in a long list of Lorentz Professors since 1955. Whose work do you admire most?
'I admire Lorentz and I greatly respect all people in the list. Wheeler, Wigner, Klein, Yang, Wess, Wilczek, Susskind, Thorne and Penrose strongly affected my work and I admire them.'

A full overview of all loans and debts between banks would help preventing a new financial collapse. But banks do not provide this information. An econophysics model by Diego Garlaschelli and collaborators reconstructs the most probable situation and finishes first More info

place in two independent tests.

The 2008 financial crisis made abundantly clear how unpredictable and vulnerable our banking system is. Banks are intertwined in a complex global web of debts and loans, where an initially local financial problem can lead to a cascade of bankruptcies. A detailed map of interbank links worldwide would enable the system to prevent dependencies from becoming too strong. However, banks do not disclose information on who they lend to and borrow from. They are only obliged to disclose their total debit and credit.

Hidden riskiness
For each bank, the lack of knowledge on how its debtors and creditors are connected to the rest of the system often implies a `hidden riskiness’. This makes it difficult to decide the interest rate for loans. To circumvent this information deficit, Leiden physicist Diego Garlaschelli and a team of international collaborators built a theoretical model, based on statistical physics, that calculates the probability of each bank borrowing money from another bank (LINK1,LINK2). His model was judged as the best probabilistic model by a collaboration of several central banks and by an independent research group.

Best model
These studies compared the performance of several alternative methods in reconstructing real privacy-protected interbank networks from partial information, and the model by Garlaschelli and collaborators was found the best one in both cases. ‘Banks determine the interest rate for loans to other banks based on perceived riskiness,’ Garlaschelli explains. ‘If bank A has lent much money to bank B, which in turn has lent money to an unstable bank C, then bank A becomes unstable as well. Our model helps noticing this, and can be used to estimate hidden risks and calculate more realistic interest rates. Correct rates keep the system stable.’

Old model
The old established model was solely based on the bare numbers for total debit/credit. For instance, to estimate the relation between Rabobank and ING, you multiply Rabobank’s total debit with ING’s credit and divide by the total sum in circulation worldwide. This creates a network where all banks are connected to each other. However this ignores the fact that in reality the majority of relations are non-existent; therefore, those that do exist are much heavier than what the old model predicts. And those links are precisely the ones that can propagate financial distress.

Density of links
Garlaschelli: ‘Besides providing a reliable estimate of which banks are connected, our model calculates the most probable weight of each relationship, depending only on one unknown factor—the density of links in the system. And because this number appears to be quite stable inside a country, we can easily proxy it and then make a prediction on which links exist, and how heavy they are. Central banks can use this information to better monitor the financial network and implement policies that prevent local instabilities from inflating into a hazard for the whole system.’

Theoretical physicists research a special class of particles; Weyl fermions. They have found them to exhibit paradoxical behavior, in contradiction to a thirty-year old fundamental theory in electromagnetism. A possible application is a new kind of electronics—spintronics. Publication in Physical More info

Review Letters.

Physicists divide the world of elementary particles into two groups. On one side we have force-carrying bosons, and on the other there are so-called fermions. The latter group comes in three different flavors. Dirac fermions are the most famous and make up all matter, including your computer screen and even your own body. Majorana fermions have recently been discovered, and might form the basis of future quantum computers. Lastly, Weyl fermions complete the group, and could find applications in a new kind of electronics: spintronics. Their weird behavior in for example electromagnets has sparked the interest of Prof. Carlo Beenakker’s theoretical physics group.

Electromagnets
Conventional electromagnets work on the elegant interplay between electrical currents and magnetic fields. Inside a bicycle dynamo, a rotating magnet generates electricity. And vice versa, moving electrical charges in a wire wrapped around a metal bar will induce a magnetic field. It seems impossible that, in addition, an electric current could be produced within the bar in the same direction. That would invoke a magnetic field around it, in turn generating a current in the opposite direction, and the whole concept would collapse.

Strange behavior
Oddly enough, Beenakker and his group have found cases where this does actually happen. Following an idea from collaborator Prof. İnanç Adagideli (Sabanci University), PhD student Thomas O’Brien built a computer simulation showing that materials harboring Weyl fermions exhibit this weird behavior. This has been seen before, but only at artificially short timescales, when the system didn’t get time to correct for the anomaly. The Leiden/Sabanci collaboration showed that in special circumstances—at temperatures close to absolute zero when materials become superconducting—the strange scenario takes place indefinitely.

Fundamental
Until now, physicists have considered this to be impossible due to underlying symmetries in the models used. That gives the discovery fundamental significance, which is also the Leiden researchers’ main drive. ‘We study Weyl fermions mainly out of a fundamental interest,’ says O’Brien. ‘Still, this research gives more freedom in the use of magnetism and materials. Perhaps the additional flexibility in a Weyl semimetal will be of use in future electronics design.’

Theoretical physicist Helmut Schiessel receives an NWO Projectruimte grant of 390,000 euro. He will use this budget to hire a postdoc and a PhD student to continue his research on flexibility of double stranded DNA.

confirmed the existence of a second layer of information in DNA, hidden in its mechanical properties. The flexibility or stiffness at specific places on a strand determines how DNA folds and consequently which information is read out and converted into the corresponding proteins.

Later he discovered a new general rule in biophysics: unicellular species—like yeast—are characterized by stiff stretches in their DNA sequence at the beginning of genes, while multicellular lifeforms—like humans, mice or zebrafish—have soft stretches instead.

The NWO Projectruimte grant enables him to further explore the subject. ‘We want to explore all possibilities of DNA shapes,’ Schiessel says. ‘For example if we simulate a spiral shape, we want to see how tightly wound it can be.’ He will also look into the influence of mechanical cues on the position of so-called nucleosomes—DNA parts that are wrapped. ‘We want to show that in our simulation we can place the nucleosomes anywhere on a whole genome by changing mechanical cues.’

This year's LION Barbecue will be held on Thursday June 22 from 5pm to 9pm.
The party committee urges everyone that has not yet registered to do so before the deadline of June 12th through this link! More info

The NWO Zwaartekracht grant of 18.8 million euro for quantum software which Amsterdam, Delft and Leiden landed collectively, means for Leiden University among others the appointment of two new permanent scientific staff members, who will each form their own research More info

group, divided over Computer Science, Physics and Mathematics.

Capacity building
‘This is exactly the right moment to invest in this kind of research,’ says Carlo Beenakker, Professor in theoretical physics at the Leiden Institute of Physics (LION) and initiator of the research proposal. ‘Ten years ago, there was little you could do with quantum computers, and in twenty years from now they will probably already be a reality. With this Zwaartekracht grant we can now build the capacity that gives us an advantage in the future, when hundreds of millions will be invested by the EU and industry. It is essential to start building a consortium now, otherwise you miss the boat.’

Machine learning
Aske Plaat is involved in the Zwaartekracht grant as scientific director of the Leiden Institute of Advanced Computer Science (LIACS). ‘The main advantage of quantum computers is the huge computing power and therefore speed. Where it takes a conventional computer years to solve a specific problem, a quantum computer could potentially solve it within one day. Computer Science in Leiden is specialized in machine learning; teaching a computer to recognize patterns. What we can already do with classical computers, we will now apply to quantum computers. That way we can perform complex calculations much faster.’

Research positions
The exact distribution of research positions within the Leiden-Amsterdam-Delft consortium will gradually become clear as the project progresses. On behalf of Leiden Physics, Dirk Bouwmeester’s group will specifically work on the development of a quantum-internet connection with Delft and Amsterdam. Beenakker sees many possibilities for interdisciplinary collaboration: ‘LIACS, LION and the Mathematics Institute are part of the consortium, but I hope also Chemistry will join. The first applications of a quantum computer might very well be in quantum chemistry.’

Vaccine development
Aske Plaat thinks of a potential medical application: ‘Imagine the amount of work involved in the development of a vaccine against a virus. You have to run through all possibilities to find the exact molecule that does the job. And in the meantime, the virus could mutate, so there is always a sense of hurry. In the future, quantum computers will be able to keep up with the pace.’

The first medium-sized quantum computers will soon be available. These computers need new software. Researchers at Leiden University have been awarded a Gravitation grant to develop the necessary software. They will be collaborating with colleagues from other research institutions.

of 18.8 million euros will enable researchers at Leiden University, QuSoft, CWI, QuTech, TU Delft, UvA and VU to carry out research over a ten-year period. Harry Buhrman (CWI, UvA, QuSoft) is the lead applicant for this Gravitation proposal. He was delighted with the award: 'Quantum compters and networks have the power to make calculations and carry out tasks that we couldn't even dream of with conventional computers. This technology has enormous potential but you have to know in advance what you're going to do with it and - even more important - how. This grant will allow us to develop the necessary quantum software. It is a huge scientific challenge but we can now really work all-out on it.'

Gigantic breakthrough
Buhrman stresses that the so-called quantum revolution is a gigantic breakthrough: 'Actually, we're at the same point as in the sixties, when conventional computers were developed. There are all kinds of fascinating opportunities ahead of us, some of which we can't even imagine right now.' The consortium will carry out research on possible applications for new materials and the development of new medicines.

Software, communication and cryptography
Dirk Bouwmeester, a physicist in Leiden, seconds this view: 'There are already quantum computers that have a limited capacity, and larger versions will be available within the foreseeable future.' Quantum technology works using quantum bits. Conventional bits are 0 or 1, but quantum bits can be both 0 and 1 at the same time. This fundamental difference opens the door to unprecedented calculations, but it also means you have to construct and programme these computers completely differently from standard computers.

Protocols
‘Our consortium will develop and implement software for small quantum computers and a quantum internet,' says mathematician Ronald Cramer (Leiden University, CWI). ‘We are developing protocols for quantum communication and for a new kind of cryptograpy that is secure in a quantum world. We will test the new algorithms and protocols on hardware that will be produced in Delft and in Leiden, and on a quantum network to be realised between Amsterdam, Delft, Leiden and The Hague.’ All this will be in addition to studying quantum machine learning that can be used to develop new materials and medicines, Aske Plaat, Professor of Data Science in Leiden, adds.

Pioneering role
Stephanie Wehner from QuTech in Delft stresses the pioneering role played by the consortium: ‘Quantum software is essential for a quantum internet. This grant means we can take bigger steps in developing and realising software applications for the future quantum internet in the Netherlands - possibly the first in the world!'

Gravitation
The Gravitation programme is funded by the Ministry of Education, Culture and Science (OCW). This year there were 37 applications for Gravitation funding. Minister Bussemaker made a total amountof 112.8 million euros available for the Gravitation programme, the aim of which is to promote world-class research in the Netherlands. The Netherlands Oganisation for Scientic Research (NWO) advised on the selection of the research groups. NWO set up an international committee of independent leading researchers with broad knowledge of scientific developments and experience with large research consortia to evaluate the proposals.

Incentive
The Gravitation programme is a vehicle for OCW and NWO to give an incentive to collaboration at the highest levels within the scientific world. Within the consortia selected, scientists from different knowledge institutions work together to set up excellent, long-term and large-scale scientific research programmes.

In the hunt for new particles, physicists look at ever higher energy particle collisions, moving the energy frontier. Some particles however are elusive not due to their high energy, but because they rarely interact. This raises a new barrier: the More info

intensity frontier.

Colossal particle accelerators like CERN’s Large Hadron Collider (LHC) are capable of inducing collisions between particles at incredible speeds. This adds enormous amounts of energy to the impact, which end up in newly created particles. When scientists look for the existence of new massive particles, they need sufficient energy to convert into mass and therefore aim to pass an "energy frontier". For the discovery of the Higgs boson, the LHC succeeded in passing first the 8 TeV and later the 13 TeV frontier.

Intensity Frontier
Leiden physicist Alexey Boyarsky looks for particles that might be lurking behind a different barrier—the "intensity frontier". Scientists suspect that some particles are so difficult to create not due to their high mass, but because they interact so rarely. This would mean that billions of collisions are needed to produce a single "feebly interacting particle". CERN’s Search for Hidden Particles (SHiP) experiment, for example, is pushing the intensity frontier forward by several orders of magnitude.

Workshop
Together with physicists around the globe, Boyarsky has organized a workshop called New Physics at the Intensity Frontier at CERN in Geneva with over 100 participants from 21 countries. ‘The main outcome of the workshop is that the intensity frontier is under-explored,’ Boyarsky says. ‘There is a lot of possible new physics to be searched for there. It is possible that with the help of these experiments, we will find explanations for the main puzzle around physics beyond the standard model: dark matter, matter-antimatter asymmetry and neutrino masses.’

Leiden physicists exploit self-assembly of small particles to someday create functional structures such as micro-robots from the bottom up. Now they took an important step forward by experimentally realizing joints on the micrometer scale. Publication in Nanoscale.

have great potential for example in medicine, as they may locally deliver drugs or perform accurate surgery. Scientists are therefore looking for ways to develop robots at this miniature scale. However, when manufacturing ever-smaller versions of functional devices, one quickly encounters limitations. Therefore, Leiden physicist Daniela Kraft works the other way round: bottom-up instead of top-down. She uses particles of around a micrometer—so-called colloids—as parts. Due to their tiny size, colloids have the additional benefit to continuously move in random directions, which allows the structures to build themselves.

Joints
While it is already challenging to create the various parts—such as cubes, triangles, and dumbbells—and combine them in the desired way, the resulting objects are usually rigid. If you dream about creating a fully functional micro-robot, you also need parts that allow movement: joints. Now for the first time, Kraft and her research group have managed to make three different types of joints at the microscale: hinges, sliders and ball joints. They publish their findings in Nanoscale.

DNA
To give their joints the necessary mobility, the researchers connect the colloids through DNA linkers. Instead of attaching the linkers to a fixed place on the colloid, they freely move across the surface. Kraft keeps the density relatively low at about a thousand DNA linkers per square micrometer on the colloid surface. That is sufficient to build in the joint functionality, while at the same time not being too many to arrest the system.

Degrees of freedom
In the macroscopic world, joints do not only create a mobile connection, they also provide functionality by constraining the motion to certain directions. A door hinge, for example, only allows the door to revolve around one direction. To impart such specific degrees of freedom and thus functionality to their microscopic joints, the physicists exploited the fact that colloids attach strongest at maximum contact. A sphere connected to a cubic particle can only slide along its side, because the contact area will diminish if it turns around the corner; this makes it a sliding joint (Figure 1b). Spheres connected to the waist of a dumbbell can only orbit around the center, as they feel maximum contact if they touch both halves of the dumbbell (Figure 1c). This provides a hinge function. Thirdly, spherical colloids can be used as ball joints because attached particles have the freedom to move in all directions (Figure 1a). These three types of microscopic joints transform rigid colloid structures into flexible ones that form the basis for future self-building micro robots.

Figure 1: Three different types of micrometer sized joints. a) Ball joints impart freedom of movement over 360 degrees for the purple spheres (movie). b) Sliding joints: the purple sphere can only slide across one side of the cube (movie). c) Hinge joints: the purple spheres can only circle around the center of the dumbbell (movie).

Prof. Cees Diks from the University of Amsterdam will give an LCN2 seminar on April 28th at 16:00 in the Science Club, titled 'Complexity in Economics and Finance'.

Abstract
We discuss how expectations feedback and heterogeneity among agents can More info

generate complex dynamics in economic and financial systems. The qualitative predictions of the resulting nonlinear models are very different from standard linear benchmarks, with important policy implications. For instance, the fundamental price can become unstable when interest rates are set too low, giving rise to multiple non-fundamental equilibria and/or global instability. This is illustrated in a central application, where we introduce heterogeneous expectations in a standard housing market model linking housing rental levels to fundamental buying prices. Using quarterly data we estimate the model parameters for eight different OECD countries. We find that the data support heterogeneity in expectations, with temporary endogenous switching between fundamental mean-reverting and trend-following beliefs based on their relative performance. Finally, a stochastic CUSP model is estimated for the same housing data to study the effect of time-varying mortgage rates in a stochastic setting.

On April 20, sixty participants will take part in LION’s triannual meeting for high school physics teachers, to deepen their knowledge of physics and astronomy and to explore new teaching methods. This edition marks a special occasion, as it is More info

the last meeting organized by Bert van der Hoorn, who will retire in November.

Astronomy group leader Elena Maria Rossi opens the evening with a lecture on her study of the Milky Way using the fastest stars in the Universe. After the dinner break, Henk Buisman will evaluate the first full operational year of his online experimental physics course KwantumWereldExperimenten. He is followed by students Thijs de Buck and Margot Leemker, who present their Bachelor thesis research on respectively Research in a living lab and High-frequency amplifiers for STM microscopy. Hannie van den Bergh, founder and creative director of Studio BH, closes the evening by explaining how her company researches and develops future materials. She works together with scientific institutes to address new developments in materials science.

Molecules are extremely hard to see in visible light, especially without using fluorescence. Leiden physicists have now made their optical technique sensitive enough to image the molecules of their interest in all sizes. Publication in Nanoletters.

bats and torpedoes have in common? They navigate by emitting sound waves and registering listening where those get absorbed or reflected. Humans do the same with light waves, except that they rely on external sources like the sun for the original emission. However when looking at something as small as a single molecule this becomes problematic, as light waves, not to mention sound waves, are bigger than the object itself.

Two light beams
In 2010, Leiden physicist Michel Orrit became the first to optically image large single organic molecules at room temperature without using fluorescence. Now, he and his group have made their technique much more sensitive, enabling them to image their subjects objects of interest—light-sensitive conducting polymer molecules—of all sizes. Just like bats, they control their own source of waves and use varying frequencies. Their first light beam has a specific color which only the targeted molecules can absorb. This causes them to heat up a bit. And because of thermal expansion this changes the refractive index of the surrounding liquid, so that a second beam will be scattered differently at the exact places where the molecules of interest are hiding.

Critical fluid
Still, this is easier said than done. Conducting polymers are quickly damaged by light, so scientists have to be extremely careful to only use very low intensities. But those are not nearly strong enough for the absorb-and-heat technique in regular liquids. Fortunately, so-called critical fluids are exceedingly sensitive to temperature changes within a small temperature range. In that regime, even the slightest heating power will alter the liquid’s refractive index by a large amount. So Orrit and his group used critical fluids and made sure temperature and pressure were precisely set during their experiment.

Locate them all
‘Until now we could only image the largest polymer molecules through absorption,’ says Orrit. ‘But because of our sensitivity enhancement, we can locate all of them. And this also gives us information on the brightness of each molecule. That is very important if you want to optimize their optoelectronic applications.’

While they are excited by a first light beam, single conducting polymer molecules heat their surroundings, leading to altered scattering of a second light beam at their location (red spots). The intensity of the signal scales with the absorption, and thus the size of each individual molecule.

Nanoparticles are widely used, but their effect on the environment is unclear because they are hard to track. Leiden physicists have developed a new method to detect conducting nanoparticles. Aquiles Carattino successfully defended his PhD thesis on the subject.

are composed of just tens to thousands of molecules, which give them vastly different properties from bulk materials built up by quintillions of the exact same molecules. For example, copper nanowires are transparent. Manufacturers utilize these special properties in products like sunscreen, tires, clothing and plasters. But their tiny size also poses a problem; they are very difficult to track, so it is still unknown to what extent they harm the environment.

Sharper contrast
Leiden physicist Aquiles Carattino and his colleagues have developed a new way to track conducting nanoparticles using fluorescence. They shine a laser on a particle using light of a particular wavelength, matching the particle’s so-called plasma resonance. Surprisingly, the particle will give off some light with an even higher energy than it received. This gives it a sharper contrast against the background.

Filter
To develop a new detection technique based on this phenomenon, Carattino used a filter to get rid of the surrounding background light. In this way, nanoparticles are easier to detect because scientists can scan for one specific, clear signal. It gives environmental researchers a method for tracking nanoparticles and investigating their effect on nature.

The European Physical Society (EPS) has awarded the Gero Thomas Commemorative Medal 2017 to Jo Hermans, emeritus Professor in Physics. He wins the award for his role as Science Editor of Europhysics News and his numerous contributions to education and More info

public understanding of physics.

The prize was created in 2000 to honour the memory of G. Thomas, who was the Secretary General of the EPS from 1973 to 1997 and played an essential role in the growth and the development of the Society. The Commemorative Medal is awarded to individuals for their outstanding service to the Society.

An essential protein that regulates our heart beat turns out to be important for cancer cells as well. The discovery might open novel treatment strategies for fighting metastasis. Publication in Science Signaling on April 4th.

we are on the tasks in front of us— as long as we live we will always remember to keep our heart beating. And although we don’t actively think about this, still it is our brain that makes sure the heart won’t even skip one beat. It controls this process among others through a protein called hERG1.

Cancer cells
Now a collaboration of groups from the universities of Leiden, Milan-Bicocca, Florence and Turin, headed by pathologist Annarosa Arcangeli (Florence) has unveiled a novel function of hERG1: it plays an important role in cancer cells too. They found that hERG1 is involved in the response of cancer cells to external mechanical signals, which are believed to be essential for metastasis. At Leiden University, physicists Thomas Schmidt and Stefano Coppola were able to provide additional evidence for this role of hERG1 in mechanical signaling. Together with their Italian colleagues they publish their results in Science Signaling on April 4th.

Drug
The finding poses the challenge of creating a drug that won't affect the heart function, but does alter hERG1’s function in metastasis. And although this will be an immense effort, nature provides drug researchers with an opening; hERG1 forms a complex with another protein called β1 integrin in both heart and cancer cells, but in heart cells also a third protein is involved. This difference might give a future medicine the means to selectively target tumors and leave the heart unaffected.

This year, the annual conference Bessensap is hosted in De Rode Hoed in Amsterdam, on Friday 16 June. Bessensap is organized by NWO and VWN (organization for science communicators) and brings scientists and science communicators together.
Traditionally the prestigious More info

Spinoza prize is awarded at this event.

Scientists are invited to submit a proposal for a 15-minute talk on their recent research.
Giving a talk at Bessensap is great exposure researchers; the majority of Dutch science journalists attends this conference. Scientists are asked to explain in a proposal (max. 200 words):
-Working title
-Subject
-Why is it interesting for media?
-What kind of presentation?
-Which domain
-Your contact info: name, email, position, research institute, research area

The deadline for proposals is Friday 31 March and can be sent to This email address is being protected from spambots. You need JavaScript enabled to view it..

You can email Physics Outreach Officer This email address is being protected from spambots. You need JavaScript enabled to view it. for any questions.

Marileen Dogterom, Professor bionanoscience in Delft and Professor molecular physics in Leiden, will take a seat at the KNAW board as of June 1st. She is appointed for four years, one day per week. Dogterom succeeds Nobel Prize winner Ben More info

Feringa, who completed his term.

Marileen Dogterom (1967) leads the Bionanoscience department of the Kavli Institute of Nanoscience at the TU Delft. Since 2000 she has also been affiliated with the Leiden Institute of Physics, initially by special appointment and later as full professor. Dogterom is one of the pioneers in the area of biomolecular physics. Her research focusses on the cytoskeleton—micro tubes that give living cells their shape and mechanical function, and enable them to divide. In her lab, Dogterom recreates parts of a cell in a controlled environment to get a quantitative understanding of the mechanics behind the cytoskeleton. Her work lays an important basis for the development of artificial cells.

Dogterom is Medical Delta Professor, member of EMBO and Academia Europaea, and received an ERC synergy grant in 2013.

systemic behavior. Examples include human cognition emerging from a network of neural cells, ecosystems from food webs, and cellular regulatory processes from protein-protein interactions. A first important question is: which agents are the ‘drivers’ of the systemic behavior? A second question is: can we detect emergent phenomena, particularly ‘criticality’ (susceptibility to small perturbations)? We address these questions using the concept of ‘information dissipation’ which we are developing. This is the idea that Shannon information is first stored in an agent’s state, and then percolates through the network due to the agent-agent interactions. I will present recent work on addressing the above questions through analytical results, computational modeling, and finally real data analysis of financial derivatives data around the Lehman Brothers collapse.

This weeks marks the 10th anniversary of the European Research Council. For the past decade, the council has contributed to many scientific projects all over Europe, including Leiden University. It has funded almost 7,000 researchers, leading to just short of More info

100,000 scientific articles. A total of fifty ERC subsidies have been granted to Leiden scientists. Five of those went to physics research groups.

Tjerk Oosterkamp was awarded an ERC starting grant in 2008. ‘That helped me to upgrade my research from atomic force microscopy (AFM) at room temperature to magnetic resonance force microscopy (MRFM) at ultra-low temperatures of milliKelvins,’ says Oosterkamp. ‘I used the grant to work at the IBM lab in California for half a year to learn the technique and to build the necessary equipment.’ In the nine years since, Oosterkamp and his research group have applied MRFM in their lab and managed to measure magnetic relaxation at the nanoscale. This is essential for getting contrast in future MRI-scans at the nano level.

Carlo Beenakker even received two ERC grants, in 2009 (Advanced) and 2012 (Synergy). ‘The Advanced grant was for studying graphene,’ he says. ‘That was only recently discovered at the time, and we had already done some prior research so we were in a good position to set up a broader study on the material’s properties.’ Beenakker obtained a Synergy grant in 2012 for building a quantum computer together with TU Delft. ‘Our aim was to build a computer composed of 1 qubit within ten to twelve years. We expected that industry would take over from there. This went faster than predicted; companies have already invested around ten times the grant money in our research. Some people ask if we could have managed without the money, but I still think we needed it to plant the seed and make it grow into the large project it is now.’

Exactly 138 years ago, Albert Einstein came into this world to forever change our understanding of it. The Leiden physics institute, where he frequently worked, organized a symposium in honor of his birthday.

enthusiasts. First of all, 3/14 gives the first three digits of the number π, evoking people to wish each other happy π-day. And as if the Universe has intended it, on this date Albert Einstein first saw the light which he later came to study so carefully. In honor of this memorable day, the Leiden physics institute organized a symposium for high school teachers and students.

The program consisted of four lectures by physics professors and two rounds of lab visits. The lectures were all related to Einstein’s work: theory of relativity, photon detection, black holes and quantum entanglement. Lab visits were organized all around the science faculty building to offer the participants a variety of options, including the Cell Observatory, the Kamerlingh Onnes lab, the mobile planetarium and the department of fine mechanics.

As the FOM Foundation officially becomes part of NWO, the various FOM prizes that are periodically awarded at Physics@Veldhoven are rebranded as NWO Physics Prizes. The deadline for submitting nominations is in less than a month: 1 April.

NWO Physics Valorisation Chapter Prize (5,000 EUR) is now open for any PhD student who gains their doctorate in the Netherlands in the field of physics. Previously, only FOM researchers could be nominated for this prize.

Individual molecules are extremely hard to see through feeble fluorescence. Tiny gold nanorods serve as new antennas to intensify their signal 500 times. Publication on February 24 in Angewandte Chemie.

of individual particles spelling out the company’s name. They had used a small electric force (STM) to see single particles for the first time. A decade later, scientists managed to actually see molecules in the human sense of the word—with visible light. They picked up on the molecules’ fluorescent light. Leiden physicist Michel Orrit was one of those pioneers.

Nanorod
Now Orrit and his group build further in that research, using tiny antennas to amplify the signal for better vision. Instead of a large metal antenna for radio waves, they use a small rod to catch visible light waves. This gold nanorod—40x80 nanometers in size—intensifies at both ends locally the electromagnetic field of light. If a molecule happens to sit at either end, it will fluoresce 500 times stronger than without the rod.

Redox
Orrit’s group publishes an article in Angewandte Chemie where they describe how they tested their concept on so-called redox reactions. These occur in any type of electric process, for example in solar cells. ‘We tested on redox reactions as a proof of concept,’ says Martin Caldarola, one of the authors. ‘At the same time, redox reactions are a very interesting application of our technique for many scientists, because they occur in so many processes.’

Midpoint
Caldarola tested the technique by determining the redox midpoint potential—a crucial indicator for a material’s ability to perform redox reactions. This gives for example its applicability for solar cells. The new method enables researchers to resolve the midpoint potential for each molecule, so they can investigate materials very precisely.

At both ends of the nanorod (yellow), the electromagnetic field of light is amplified by a factor 500 (red spots). If a molecule (blue dots) is situated there, its fluorescent signal is also 500 times stronger.

Newspaper NRC and popular scientific magazine New Scientist write about Corentin Coulais’ research on one-way-traffic metamaterials. He published a paper on this subject in Nature on February 13. Click here to listen to Coulais talk about More info

The Materials Research Society (MRS) has awarded Prof. Joost Frenken with the 2017 Innovation in Materials Characterization Award. The society hands out the prize in honor of an outstanding advance in materials characterization that notably increases knowledge of the structure, More info

composition, in situ behavior under outside stimulus, electronic behavior, or other characterization feature, of materials.

Frenken was chosen from a large group of nominees, for
"the development, application and commercialization of high-speed, temperature-controlled, in-situ scanning probe microscopy, leading to key insights in the structure, dynamics and chemistry of surfaces and interfaces."

Jacco Wallinga from the Leiden University Medical Center (LUMC) will give an LCN2 seminar on February 24th in the Science Club, titled 'Infection Dynamics: using theory and data to reconstruct the causal structure of epidemics'.

diseases are increasingly used to design and evaluate infection control strategies. The infectious disease models describe (or approximate) a contagion process on a given network structure. The dynamical behaviour of such models is often rich and complex. In rare situations we have observations on the network structure and we are interested in the spread of infection over this network. An example is the referral network of patients between hospitals, which allows us to model the spread of resistant bacteria between hospitals. In most situations we have observations on the infected persons (time of symptom onset, place, demographics) but we have no information on the underlying network structure (the ‘background’). We are interested in interpreting these observations in a ‘background independent’ manner without making strong assumptions about the network structure. Our approach is to use the observed infection events to reconstruct the causal structure of the infection network. We discuss how we can use this approach to make sense of observations during emerging epidemics.

Physicists have developed a material that breaks one of the fundamental principles governing many physical systems. Ordinary materials transmit external forces equally, no matter where the pressure comes from. The newly developed material breaks this rule and could potentially be More info

of interest in soft-robotics or shock absorption related applications. The research team from AMOLF, Leiden University and the University of Texas at Austin published their findings on February 13 in Nature.

Reciprocity
The researchers’ breakthrough lies in the ability to overcome reciprocity in mechanics. Reciprocity governs how light, radio signals, sound and motion travel through materials. It ensures that when you are able to send a signal you can also receive it. Reciprocity also means that, if pushing a soft object on the left side results in moving the right side by a certain amount, we can expect the same motion at the left when pushing the right side.

The material
Corentin Coulais (AMOLF/Leiden University), Dimitrios Sounas and Andrea Alù (UT Austin) have developed the first kind of non-reciprocal material to break this rule. The material can transmit motion in one way and block it in the other. Coulais says: “We created and fabricated so-called metamaterials, which acquire their extraordinary behavior from their architecture: a flat rubber sheet patterned with carefully designed holes. Each unit in this design slightly leans rightward, which is an important feature to break reciprocity.

How does it work?
However subtle the metamaterial design may be, when put to the test, it exhibits drastically different responses. Applying pressure on the right side of the metamaterial easily induces motion within the structure, but only in the near vicinity of the pressure point and the effect on the other end is negligible. However, applying the same amount of force on the left side clearly has a much stronger effect as the motion propagates through the entire material. The researchers further discovered how to make the metamaterial sensitive enough to show non-reciprocal behavior even under minor pressure.

Towards non-reciprocal devices
'Non-reciprocal metamaterials are currently being applied to manipulate radio waves and optical communications devices, but have remained so far unexplored for mechanical motion,' Alù says. 'Our research opens up new strategies to block or transmit vibrations and shocks depending on where they come from.' The non-reciprocal mechanical metamaterials could also be further leveraged for various applications, such as soft robotics, prosthetics and energy harvesting. The team is looking forward to explore ways to control the degree of non-reciprocity in real-time as the material is used for the application of choice.

Header image
This picture shows a topological mechanical metamaterial that features large non-reciprocity: the right side moves when pushing on the left, but the left side moves much less when pushing on the right with the same force.

Single-celled organisms have stiffer DNA than multicellular lifeforms like humans and rice. Theoretical physicists managed to simulate the folding in full genomes for the first time to reach this conclusion. Publication in Biophysical Journal on February 7.

DNA stores information using four base letters, which encode for the proteins that arrange all processes in our body. With a length of about two meters each, DNA molecules need to cozily curl up to fit inside our tiny cells. Local variations in stiffness allow for less or more wrapping, affecting the readability per set of letters. These mechanical cues in genes provide a second layer of information, as Leiden physicist Helmut Schiessel theoretically demonstrated earlier.

General rule
Now in a follow-up research, Schiessel and his colleagues have improved their computer model, allowing them to calculate folding in the full genome of fifty different species. This led to the discovery of a new general rule in biophysics: unicellular species—like yeast—are characterized by stiff stretches in their DNA sequence at the beginning of genes, while multicellular lifeforms—like humans, mice or zebrafish—have soft stretches. ‘Unicellular species need access to all their genes in the one cell they have,’ Schiessel explains. ‘Therefore it needs stiff DNA so it is easily read out. Humans have many different types of cells and we find that this is reflected in soft DNA.’

Full genome
To make their discovery, the researchers averaged over many simulated gene sequences and looked at the stiffness right at the beginning of a new gene. Comparison with real-life experiments shows that the model works. Schiessel: ‘Our simulation is much faster than experiments, so we were able to analyze for the first time the full genome of several lifeforms. You would think from our finding: “the softer the DNA, the more complex the lifeform,” but funny enough we found rice to have the softest DNA of all the species we analyzed.’

The researchers modeled DNA wrapping in 50 different lifeforms. The x-axis spatially represents 2000 base pair long stretches of DNA. The nucleosome occupancy (y-axis) is a measure for the likelihood of the DNA to be wrapped. Large values indicate soft DNA. Rice appears to have softer DNA than for example humans.

Physicist Joost Frenken was interviewed on the radio show 'Langs de Lijn en Omstreken' (Radio 1) about the recent discovery of superconductive graphene. The one-atom-thick material was already known for its strength, flexibility, lightweight and good conductivity.

On the evening of Tuesday January 31st, the physics institute organizes its triannual meeting for over seventy high school teachers. The program consists of lectures on developments in modern physics and the latest initiatives in the world of education.

Bouwmeester and Anthony Brown will report first-hand on the current state of quantum research and the Gaia space mission. Vice dean of the science faculty Han de Winde talks about the differences and collaboration between the studies Physics in Leiden and Applied Physics in Delft.

Education
Henk Buisman and Wim Sonneveld conclude the evening with the latest news on school projects. Buisman presents the progress of his project Kwantumwereld Experimenten, which has been set up to support the new quantum domains in the physics curriculum with a series of experiments. Sonneveld introduces a ‘briefcase’ filled with practical experiments on Geophysics—a relatively new subject in high schools.

Jos Rohling from the Leiden University Medical Center (LUMC) will give an LCN2 seminar on January 27th in the Science Club, titled 'Dynamical networks of the biological clock'.

Abstract
Understanding how neurons and brain regions communicate, coordinate, synchronize, and More info

collectively respond to signals and perturbations is one of the most intriguing, yet unsolved problems in neuroscience. We investigate one brain area that is involved in time regulation of the body. This circadian clock, which is located in the suprachiasmatic nuclei (SCN) and drives the daily 24-hour rhythms in our body, is functionally dependent on emergent network properties. While the ability of individual SCN neurons to produce 24-hour rhythms is a cell-autonomous property, the ability of the SCN to respond to light, to adjust to the seasons and to synchronize after a jet-lag is critically dependent upon the state of the neuronal network. The synchronized network output regulates all daily rhythms in our body and is heavily dependent on the interactions between the neurons and the network topology of the clock. We investigated methods to assess network properties on our data, such as small-worldness and scale-freeness for this cellular network.
We have observed that temporal behavioral patterns and the central clock show scale invariant behavior. With disease and aging, scale invariance is lost, and also in a brain slice preparation when the clock is not communicating with other brain areas, scale invariance is absent.
Currently we investigate how we can use our data to extract the network topology for the SCN. A complicating factor is that this functional network is not static during its oscillation cycle: for example, the network appears to be different between day and night time. This dynamical nature of the network is normally not taken into account in network studies. I will discuss our latest results, highlighting the advances that were made in recent years, but also discussing the challenges we still face in this field.

Special amplifying agents can make MRI scanners and NMR techniques hundreds of times more sensitive. Leiden physicists have now found a method to test their efficiency. More sensitive MRI scans could for example improve our understanding of cystic fibrosis or More info

MRI scanners are a useful tool for medical doctors to look inside the human body. It allows them for example to easily check for torn ankle ligaments or study lungs by having patients breath in xenon gas, while xenon is clearly visible on the scan. Sometimes however, the defect is so well hidden that the MRI scanner is not sensitive enough.

Mysterious collaboration
Leiden physicists research amplifying agents, which increase the sensitivity of MRI scans and NMR, with a method called dynamic nuclear polarization (DNP). Those agents collaborate with the substance that is actually scanned—like xenon—but how this happens exactly is still very unclear. Dr. Martina Huber and her research group have now developed a method to explore this mysterious collaboration. ‘Nowadays scientists look for new DNP agents through trial-and-error,’ says Huber. ‘With our new method we can start to understand how agents work, and then we can eventually make much better predictions on which substances would be good agents.’

EPR
To research DNP agents at the molecular level, Huber studies them with extreme high-field Electron Paramagnetic Resonance (EPR). With MRI itself they are of course invisible; otherwise they would cause noise instead of being a utility. To test their method, the researchers characterized the agent AMUPol, and published the results together with Prof. Marc Baldus’ group (Utrecht University) in the journal PCCP.

Rare proteins
MRI scans with a hundred times more sensitivity could for example detect rare proteins in the human body, enabling scientists to better understand why they stop functioning in patients with cystic fibrosis, Parkinson’s, amyloidosis or Alzheimer’s. The DNP method already proved to be highly successful by exposing many details in solid state physics NMR research.

Because the last Elfstedentocht celebrates its twentieth anniversary, New Scientist spoke with physics professor Tjerk Oosterkamp about his research on the slipperiness of ice and his passion for skating on natural ice.

At the new year’s reception, Scientific Director of Casimir Tjerk Oosterkamp handed out the 2016 Hendrik Casimir Prize to physics students Andrea Peña (Leiden) and Jorrit Hortensius (Delft). The Casimir Research School yearly awards the Hendrik Casimir Prize to the More info

best MSc students in (Applied) Physics of Leiden University and TU Delft. The prize consists of a certificate and a sum of €750.

The prize is based on the revenues from a donation by the late Josina Casimir-Jonker, wife of the famous Hendrik Casimir. Peña and Hortensius were nominated by a committee formed by Jos Thijssen (director of Master Education Delft), Martin van Exter (Director of Education Leiden), Hara Papathanassiou (study advisor Leiden) and Christophe Danelon (coordinator of the Casmir pre-PhD Master program in Delft). They wrote the following about the two MSc students.

"Andrea is a talented student who has performed very well in her first year. Andrea has first joined the Physics program as an Erasmus exchange student from Spain. She adapted remarkably well and even followed a MSc course and carried out a successful research project in her BSc. She has a low key approach and proceeds carefully, delivering high performance while never overstating her capacity or achievements. Moreover, she is a very friendly person, always ready to help and share her experience with others. In the current semester she has been carrying out a project at ARCNL (local supervisor D. Bouwmeester) on investigating potential correlation of the amplitude of THz pulses from a two-color laser with the excited modes of an air-plasma mix."

"Jorrit completed a double bachelor programme in Applied Physics and Applied Mathematics with both diplomas awarded ‘cum laude’ (with distinction). His performance in the master programme is equally stellar. Jorrit usually does his coursework with a group of about five very good students in the master Applied physics and these students all have an exemplary working attitude which, together with their talent for physics and math, has a stimulating effect on a wide fraction of their cohort. Jorrit emanates enthusiasm for physics and for the courses he takes. He participates in activities of the student physics society VvTP."

For many years, scientists have observed a correlation between Alzheimer’s disease and a surplus of iron in the brain. However, a causal link between the two has not been proven yet. We lack knowledge concerning the specific form of iron More info

Iron intake
We eat daily between ten and twenty milligrams of iron. After being absorbed by the gut, this metal participates in a wide variety of essential metabolic processes, including oxygen transportation, DNA replication and electron transport. However, if it’s not properly regulated, iron can cause damage to cells.

MRI
In the brains of patients with neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease, iron is upregulated: it can increase to up to three times greater than the normal level. Scientists can use MRI scans to indirectly image iron in post-mortem brain tissue, but the data interpretation remains much debated. Such measurements are indirect, affected by several sources of artifacts, and poorly quantitative.

Magnetism and biology
A multidisciplinary team of Leiden physicists (LION) and radiologists from the Leiden University Medical Centre (LUMC), led by postdoc Lucia Bossoni, has developed a method based on the combination of Electron Paramagnetic Resonance Spectroscopy (EPR) and SQUID magnetometry to detect and quantify complementary species of iron in post-mortem brain tissue. In combination with MRI, this approach will allow scientists to better understand the role played by iron in Alzheimer’s disease.

X-band EPR spectra of brain tissue. The graphs show a comparison between Alzheimer’s disease tissue (left) and healthy control tissue (right). The potentially toxic iron appears as a band at a magnetic field of about 1.5 kG.

In the latest episode of DWDD University on 'Light', Prof. Robbert Dijkgraaf used a number of experiments from the Leiden Physics Practicum Lab. He showed why the sky is blue, that light is a wave phenomenon, how magnetism and electricity More info

are connected and that white light consists of all colors of the rainbow.

Materials are either gas, liquid or solid, based on how their molecules respond to temperature and pressure. But what if the building blocks are self-spinning particles instead of ordinary molecules? Theoretical physicists found out what determines the phase of those More info

When water reaches 100 °C, it turns into a gas phase. At sea level, that is. If you take away some air pressure, water will boil already at colder temperatures. It is clear that materials made up of ordinary molecules take on a phase depending on temperature and pressure. Leiden theoretical physicist Prof. Vincenzo Vitelli wondered what happens if materials have self-spinning dimers as building blocks instead.

Simulation
To this end, first authors Benny van Zuiden and Jayson Paulose simulate self-spinning dimers on their computer and study how they organize themselves. When they apply a gradually increasing pressure on them, they see the system change from an ordered state to a very chaotic state.
In the figure below we see on the left a beautifully ordered state, with dimers neatly forming a triangular crystal lattice. Moreover, the relative orientation of nearby particles are locked as they spin.
At the far right, the concentration is so high that the system gets stuck in a glassy phase. Remarkably, there is a liquid phase in between. Usually a substance will become more solid as its density increases. Here the opposite happens.

Liquid
So how can there be a liquid state? With low density, the dimers have plenty of room to move as they wish and stay in sync, like a group of stage dancers. When the stage is too small, dancers will bump in to each other and they chaotically move around, as particles in a liquid. If the stage however gets so tiny that dancers are unable to move, they get stuck in a disordered configuration reminiscent of a glass.

Graphene and other layered materials combine into completely new substances. Leiden physicists establish the ground rules for designing such materials by measuring how the layers in the stack interact. Publication on November 29 in Nature Communications.

of graphene—a single layer of carbon atoms—have been widely celebrated. Not only does graphene exhibit remarkable physics, it also shows great promise for new applications, like flexible display screens and solar cells. But scientists aren’t easily satisfied. The hunt is on for the next generation materials: layered stacks composed of single sheets of ‘flat’ materials like boron nitride (BN), graphene (C) or tungsten disulfide (WS2).

Sum of parts
The trick is that such a layered cake is not just the sum of its parts. You might get properties completely different from those of the individual layers. This even goes for two layers of the same sort; bilayer graphene is in no way like its monolayer cousin. It all depends on how the layers interact. Leiden physicist Sense Jan van der Molen and his group have developed a method to figure out the rules of the game. They can now determine the interaction between layers in each combination of materials.

LEEM
Using a technique called Low-Energy Electron Microscopy (LEEM) they shine electrons of very low energies at a sample. For every energy level, they record an image of the surface, telling them how many electrons are reflected. This gives them all the necessary information to determine the interlayer interaction and therefore the properties of the newly created material. Their method resolves details 100,000 times smaller than other techniques. This is crucial because novel nanomaterials are typically extremely small—less than the thickness of a human hair.

Tailored
‘We used our method to prove that boron nitride and graphene do not interact with each other as was only assumed so far,’ says first author and Veni fellow Johannes Jobst. ‘But more importantly, it shows the potential of this novel technique. Now we can study any other combination of layers, like semiconductors on graphene, or two different semiconductors. And once we understand how this interaction works, we can freely design materials that are tailored to specific needs.’

Johannes Jobst is nominated for Discoverer of the Year at Leiden University’s science faculty.

Leiden physicists study stacks of layered materials using a novel technique. They can now answer the question whether a given stack of various materials has properties different from its constituents by probing the interlayer interactions. They employed this method to verify that graphene (grey) interacts strongly with graphene, and boron nitride (purple) interacts strongly with boron nitride, while graphene is not influenced by the presence of boron nitride. We see on the upper right the resulting material: different properties (shades) for combined graphene + graphene and boron nitride + boron nitride, but no interaction between graphene and boron nitride. On the bottom right we see a hypothetical state where all layers interact to form a completely new material, which is not the case in this example.

Graphene holds the promise of impressive applications such as wear-resistant, friction-free coatings. But first manufacturers have to be able to produce large sheets of graphene under precisely controlled conditions. Dirk van Baarle studied how graphene grows at the atomic scale More info

and what determines the friction with other materials. PhD defence on 29 November.

Predictable quality
An almost perfectly friction-free, wear-resistant coating in machinery could generate enormous savings in fuel and maintenance. In the world of nano-technology such coatings will probably even have applications that we are currently not able to predict. In his PhD research Dirk van Baarle studied a candidate for such coatings: graphene. Van Baarle: 'It's quite a challenge to produce graphene of a predictable quality.'

Graphene is only super strong if the wire mesh of carbon atoms that make up the material are perfectly regular in form. But with the present production methods, a sheet of graphene is in practice almost always made up of a patchwork of small pieces that have been grafted onto one another. Van Baarle was able to observe almost per carbon atom live how islands of graphene grow towards one another and how this process is influenced by temperature and substrate. This is the first step towards a production method for making larger, flawless sheets of graphene.

Chicken wire pattern
Graphene occurs spontaneously when a very clean surface of iridium comes into contact with ethylene (C2H4, a hydrocarbon) at a temperature of around 700 degrees Celsius. The gas molecules disintegrate on the hot surface, leaving behind the carbon atoms, which spontaneously form a network of linked hexagons, in a chicken wire pattern.

For his research Van Baarle used a unique piece of equipment in the Huygens-Kamerlingh Onnes Laboratory, the VT-STM (Variable Temperature Scanning Tunneling Microscope). This apparatus comprises a minuscule stylus with a point that is just a few atoms thick. It can be used to systematically scan a surface with such a high degree of precision (what you are in fact doing is measuring the flow of electricity between the stylus and the surface) that even individual atoms can be distinguished. What makes the Leiden instrument unique is that it can do this even at high and variable temperatures.

A remarkable finding is that atomic processes occur not only in the growing layer of graphene. In practice, the surface of the iridium does not match the atomic layers in the substrate perfectly. The iridium forms broad steps on the surface, where the graphene grows over it. But these steps can continue to grow underneath the graphene or can withdraw as a result of the iridium atoms in the substrate realigning themselves. This process, too, has to be closely controlled in order to allow perfect sheets of graphene to form.

Contact points
In the theoretical part of his research, Van Baarle developed a model of how friction occurs at atomic level. When two surfaces slide over one another, the actual contact points are only nanometres in size, just a very few atoms. The friction is at its maximum when the stiffness of the nano-protrusions is roughly average: not too soft, but also not too stiff.

Van Baarle: 'One of my colleagues is currently coating an object with nano-needles using a lithography technique (a technique that is also used for computer chips). These needles vary in stiffness, depending on the direction in which they bend. This means that the friction of the surface is different in different directions.' This can be useful, for example, for a coating on a revolving axis, to prevent it moving laterally.

'Internally we are already using graphene coatings in our equipment to reduce friction without using lubricants,' Van Baarle explains. 'It has already resulted in a patent and a start-up, Applied Nanolayers. No wonder our professor, Joost Frenken, has already won a valorisation prize.'

The Royal Holland Society of Sciences and Humanities (KHMW) has awarded Leiden Physics PhD student Anne Meeussen the Shell Graduation Prize in Physics, which comes with 5,000 euro in prize money. Every year, KHMW distinguishes young talents with the aim More info

to promote scientific education in technical subjects. Meeussen wins the graduation prize for her research in designing new materials with specific behavior. Recently, she published a paper in Physical Review X.

Casimir PhD student Eduardo Pavinato Olimpio won the Pfizer Prize for Life Sciences, also at a value of 5,000 euro.

Leiden Bachelor student Remi Claessen received a Young Talent ‘Aanmoedigingsprijs’ for Physics and Engineering Physics, including a cash prize of 500 euro.

During the month October, LION organized its second annual Image Award. The organizing committee received many beautiful contributions. A big thanks to all who have worked hard on sending in their image! After long consideration, the jury finally decided on More info

a top 3 of wonderful science pictures. Congratulations to all!
The winner will receive the first prize at the Ehrenfest Colloquium on December 7th. The winning image will get an honorary spot on the 1st floor of the Oort building.

Winner
Vera Meester

Scanning electron microscopy image of micron-sized particles, so-called colloids, with a smiley face on one side of the sphere. The particles are made by solidifying soft spheres, which induces buckling of the outside of the shell resulting in dents. In this particular case, the dents form smiley faces on al particles! These particles are used in self-assembly experiments to obtain larger structures with novel physical properties.

Runner up
Casper van der Wel

Lipid membranes mediate a force between membrane proteins. In this confocal microscopy image, six spherical lipid membranes together with many 0.001 mm sized particles can be seen. Several of these particles are attached to the membranes: these particles attract each other through the membrane deformation they induce.

Third Place
Martin de Wit

Optical microscope image of a detection chips covered with silicon nitride used for Magnetic Resonance Force Microscopy (MRFM), a technique in which we detect signals from a small number of spins using a mechanical resonator. The horizontal line is the RF-wire, used for the generation of magnetic fields to manipulate the spins in the sample, while the two small squares in the center are used to inductively detect the motion of the resonator. The visible colored rings are the result of the gradually reducing thickness of the silicon nitride layer.

The Leiden Institute of Physics has been awarded three out of twelve available grants from the FOM Projectruimte. Principal Investigators Milan Allan, Stefan Semrau and Carlo Beenakker all receive around 400,000 euro for their research.

for his research on ‘Visualizing the emergence of high-temperature superconductivity using the spin Hall effect’ . With his research group he wants to map the so-called spin character of materials that are superconductive at relatively high temperatures. He tries to do this with a new atomic probe. In his lab, Allan manipulates materials by adding chemical doping, in order to transform them from insulator to conductor, and possibly a superconductor. With his research on spin character, he aims to understand the influence of chemical doping.

Tumor
Stefan Semrau wrote a successful proposal on ‘Understanding cancer through physics: is tumor initiation a phase transition?’ He will use the grant to research a new theory on the transition from normal cellular growth to cancer cell growth. For this, he studies lab models of tumors. To conduct his research, he has to first develop a new imaging technique.

Quantum transport
Carlo Beenakker receives subsidy for ‘Quantum transport in Weyl semimetals’. These Weyl semimetals have only been discovered recently. They are the three-dimensional counterpart of graphene, with massless electrons inside. Beenakker and his research team will theoretically explore the electrical transport properties of Weyl semimetals.

Abstract: The symmetry protected Dirac spectrum near the charge neutrality has been a source of exotic physical behaviors observed in graphene. In a disorder-free clean sample, attained by encapsulating graphene in More info

between hexa-boron nitride, we can tune the Fermi energy to be much smaller than any relevant energy scales. In this presentation, we will discuss several examples where extremely small Fermi energy plays a critical role. In the first example, we present the Andreev reflection realized in graphene in contact with van der Waals superconductor NbSe2 with a clean interface. By reducing the Fermi energy of graphene smaller than the superconducting gap, we observe the evidence of a long-sought specular Andreev reflection, where the superconducting Cooper pairs split into a pair of electron and hole across the charge neutrality point. In the second example, we will discuss a drastic enhancement of the electronic thermal conductivity at the charge neutrality as the Fermi energy becomes smaller than thermal energy. We report a strong violation of the Wiedemann-Franz law at the charge neutrality, suggesting the realization of the relativistic hydrodynamic transport. Finally, we will also discuss our recent experimental results on quantized Hall-drag resistance in double bilayer graphene system where evidence for the exciton condensation phases across Landau levels with different filling factors can be inferred.

Please Note: The Colloquium Ehrenfestii takes place on Wednesday evenings starting at 19:30 hours in the main auditorium of the Oort building. Before the Colloquium, there is a common dinner in the canteen located on the ground floor of the Oort building. This dinner starts at 18:00 hours sharp and is FREE of charge, under the condition that one attends the colloquium and that one has made a reservation before noon on the Tuesday preceding the colloquium.
The registration form is only accessible from within the University (to block spammers); if you would like to register for the dinner from outside the University, please send an email to fran [at] lorentz.leidenuniv.nl. The revolving doors to the Oort building are accessible until 9 pm. After that, they are locked and one should instead use the sliding doors directly adjacent to the Huygens building: these can be opened with an electronic key.

A specifically designed collection of gears is soft on one end and rigid on the other. These are robust properties of the system that hold even in the presence of manufacturing imperfections. This emerging research area may lead to new More info

Gears
Image you have two connected gear wheels. If you turn one clockwise, the other will turn counterclockwise. Now what if you connect another gear to both of them, to form a circle? The system will get stuck. Leiden physicists Anne Meeussen and Jayson Paulose now have developed a complex structure of gears that is stuck in one place, but loose in another. If you consider the whole structure as a new (meta)material, it is rigid on one end and soft on the other.

Mathematics
In the video below, this remarkable mechanism seems like magic, but the researchers have actually devised it from mathematics. ‘The beauty of this principle is that it’s a robust system,’ says group leader Prof. Vincenzo Vitelli. ‘We can decide which parts are soft or rigid, and the mechanism keeps working even if the gears are imperfect. This property is often called topological robustness.’

Inherent
Because the rigidity properties are inherent to the system, manufacturers can use the theory to build mechanical devices like watches using cheaper components, while preserving performance. Vitelli: ‘This may be best applicable to tracking devices, like satellite trackers that are based on geared mechanisms’.

Topological insulators
The theory is inspired by electronic topological insulators, which earned the 2016 Physics Nobel Prize. Those are insulators on the inside, but conduct electricity on their surface. And even if they have imperfections, the current will keep flowing. Instead of electronic properties, Vitelli’s group addresses rigidity. His systems are rigid in selected places and soft in others, irrespective of imperfections.

Luca Avena from the Leiden Mathematics Institute will give an LCN2 seminar on November 25th at 16:00 in room HL214, titled 'Epidemic Spread on Networks'.

Abstract
I will discuss some probabilistic tools and related randomized algorithms to explore the More info

architecture of a network.
The two main applications will be 1) to identify well distributed nodes/individuals in a network and 2) to construct a multiscale procedure giving successive coarse grained descriptions of an arbitrary signal defined on the network.
As an example of the first application, given an electricity grid, we can build a randomized algorithm to place the power plants in the grid giving on average the most efficient energy distribution.
As an example of the second application, we can efficiently compress a signal (i.e. a data set or a function) on the given network.
These applications are based on properties of two main mathematical objects: a random spanning rooted forest and a related random surfer to explore the network.

Joint work with Fabienne Castell, Alexandre Gaudilliere and Clothilde Melot from Marselle, France.

On November 24, the 4th edition of the Gorter Symposium will be held at the "Rijksmuseum van Oudheden". The symposium, which is organized by the Gorter Foundation, has brought the different research groups on Magnetic Resonance from Leiden together for More info

the last ten years. This year’s edition will highlight the latest achievements from the different disciplines and includes the Gorter Lecture by David Britt from UC Davis.

The program starts at 12:30 and is free of charge, but we do ask you to register by sending an e-mail to Rijsewijk [at] Physics.LeidenUniv.nl.

A collaboration of physicists and chemists organized the LEELIS conference on new computer chip technology in Amsterdam on 10-11 November. Leiden physicist Joost Frenken is director of the organizing institute ARCNL.

our computer chips double their amount of transistors.. To keep up with this law, scientists and engineers face increasingly more challenges as sizes get ever smaller. Currently they are confronted with the task to produce chips with transistors smaller than 13 nanometer. For this, they will need to re-invent their technology. On a two-day conference in Amsterdam, organized by scientists from ARCNL and Leiden University, physicists and chemists from around the world got together to discuss their progress.

Imprint
Chips are produced in a similar way to how photographs used to be made. A mask ensures that light illuminates certain areas of a photosensitive material, causing local chemical changes there. This way, the mask imprints a desired structure onto the material. The smaller the light’s wavelength, the more accurate this imprint will be, eventually allowing for smaller transistors.

Extreme ultraviolet
For the next generation transistors, industry needs to find ways to actually use smaller wavelengths. Billions of euros are currently invested in a technique with 13-nanometer light (92 eV), called extreme ultraviolet (EUV). This huge sum is needed because EUV requires new technology on many different levels. We know little about how the photosensitive material responds to EUV light. For example, the light will generate an electron cloud inside the material that may blur the detailed imprint.

Electron cloud
Leiden physicist Sense Jan van der Molen gave an invited talk at the LEELIS conference on his research on electron clouds. Together with Aniket Thete (ARCNL/Uni Leiden) and Ruud Tromp (IBM/Uni Leiden), he studies electrons of varying energy and looks at the impact they have on photosensitive material. Thus they can pinpoint exactly for which electron energy the imprint gets blurred. Such data are crucial for the progress of EUV-lithography as the chip-defining technique of the future.

The conference on Low-energy electrons: Lithography, Imaging and Soft Matter (LEELIS) is organized by the Advanced Research Center for Nanolithography, an institute founded by ASML and FOM.

Sixty-four high school students from Slovenia, Czech Republic and The Netherlands have visited Leiden University in the week of 7-11 November, as part of the Talent Education Project. They split in eight groups, of which some went to the science More info

faculty to conduct research projects.

The exchange program marks the launch of the students’ final thesis, which they write in their fifth year of high school. Other events are hosted by Brno, Pilsen and Ljubljana, which together with Leiden form the home towns of the participants.

One group of eight pupils was asked to think about the question: ‘What are the technological and societal consequences if tomorrow we’d discover super conduction at room temperature?’ This resulted in interesting applications, like protection from cosmic radiation for human space travel and easy energy storage in coils, for example induced by solar power in the desert. Apart from this assignment, the students worked on fun experiments with Stichting Rino and got a tour at the department of fine mechanics.

Another group was challenged with the case of how to communicate the Universe’s accelerated expansion with a young audience. First, they derived that this acceleration is indeed happening, from actual astronomical data of type 1a supernovae. Next, the students developed several prototypes of communication strategies. For example, by placing galaxies on a rubber band and stretching it, they mimic the expansion in one dimension. Later, they will extend this outreach concept to two dimensions and create movies and animations.

Electrons that spin synchronously around their axis, turn out to stay superconductive across large distances within magnetic chrome dioxide. Electric current from these electrons can flip small magnets, and its superconductive version could form the basis for a hard drive More info

Super current
In Leiden in 1911, Nobel Prize winner Heike Kamerlingh Onnes discovered the principle of superconduction; electric current flowing through ice-cold metal without any resistance. With this super current you can transport electricity or run an electromagnet without energy loss—an essential asset for MRI scanners, maglev trains and nuclear fusion reactors.

Pairs
Half a century later, electrons appeared to form pairs, enabling the (super) current to escape the classical rules of electricity. Physicists assumed that both electrons spin around their axis in opposite directions, so that the pairs have a net ‘spin’ of zero. Around the turn of the century, that assumption proved to be premature. Super currents can indeed have a net ‘spin’, and with that possibly manipulate small magnets.

Hard drive
Leiden physicist Prof. Jan Aarts and his group have now created a wire made of chrome dioxide, which only carries currents with ‘spin’. They cooled it to a superconducting state and measured a particularly strong current of a billion A/m2. That’s powerful enough to flip magnets, potentially facilitating future hard drives without energy loss. Moreover, the super current covered a record distance of 600 nanometer. This seems like a small stretch—bacteria are bigger—but it lets electron pairs live long enough to work with.

Electron microscope image of a chromium dioxide devices based on wires. The green wire is the chromium dioxide ferromagnet. The orange wires are superconductors and are necessary to produce a superconducting current through the green wire.

Cosmologists have many possible models for the Universe, of which only one can be true. A new flowchart will eliminate some of them when two specific Universe features are accurately measured. Publication in Physical Review D on 7 More info

November.

Cosmologists try to understand how the entire Universe formed and evolves. In short, cosmology is the science of everything, except for that pale blue dot floating around in the vastness of space. By aiming their telescopes at distant galaxies and the afterglow of the Big Bang, cosmologists look back in time and pick up pieces of the puzzle. They put these pieces as parameters in the many possible models they have created for our Universe. The more parameters are precisely measured, the more models can be excluded.

Perturbations
Lately, astrophysicists have done many observations to measure two specific parameters. These are called μ and Σ. They represent how fast galaxies formed from the irregularities in the Universe just after the Big Bang and how much distant light is bent by gravitational lensing. Recently measured values of μ and Σ generally show a mild tension with the leading cosmological model called ΛCDM. More and better observations will zone in on an accurate value of the two parameters. Things would get very interesting if the observed μ and Σ indeed did not agree with the values expected in the popular ΛCDM model. So then what?

Flowchart
Leiden University physicist Alessandra Silvestri and Levon Pogosian from the Simon Fraser University publish a paper on in Physical Review D with an overview of all the models we can rule out in case of each value that will be measured. In a flowchart they answer questions with ‘yes’ and ‘no’, leading to the subsequent conclusion. For example, if μ is greater than one and Σ is less, then a large collection of models is ruled out; the so-called Horndeski class. With their paper, they add more significance to future cosmological observations, as it puts concrete meaning in the measurement of otherwise abstract values.

LCD screens use liquid crystals which have a high degree of order, even though they form a fluid. A new theory maps out the interplay between order, temperature and symmetry. Publication in Physical Review X.

you’re staring at a collection of liquid crystals right now. Most screens nowadays are LCDs, meaning that they have a number of liquid crystals in every pixel. The computer determines for each pixel whether it blocks light or let it pass, by sending small currents through the liquid crystals. That way the correct color filters give the pixel the appropriate color.

Temperature
LCD technology needs a certain amount of order. If the screen gets too hot, the liquid crystals will convert into a useless, chaotic, ordinary fluid. At room temperature they also form a fluid, but they have the necessary degree of orientation order. Leiden theoretical physicist Prof. Jan Zaanen and his group now formulated a theory on the interplay between symmetry, order and temperature.

Symmetry
The more symmetric a liquid crystal is, the colder it needs to be to still contain sufficient order. After all, it is harder to notice a crooked part in a composition with symmetry in many directions. In the figure below we see the ordered state on the left—displayed in purple in the graph. With low symmetry (left in graph) it abides even at high temperatures, but with high symmetry (right in graph) this takes extreme cold.

The figure on the left displays the ordered state. In the graph this is represented in purple. With low symmetry (left in graph) the ordered state abides even at high temperatures, but with high symmetry (right in graph) this takes extreme cold.

LION organizes a course on quantum mechanics for high school Physics teachers. As the curriculum gains a new quantum component, Leiden University enables teachers to give the best possible guidance to their students.

through the new quantum syllabus and provide additional background information, to refresh and enrich teachers’ memory on the subject. They address topics like particle properties of waves, atomic structure and Eigenvalues.

Alongside lectures, the participants engage in workshops and exercises to master their new curricular material. ‘We assume that teachers have a solid sleeping knowledge of quantum mechanics, that they didn’t use until one year ago, when quantum physics was introduced in the high school program,’ says Bert van der Hoorn, co-organizer with Henk Buisman. ‘The course aims to get teachers right on top of the material so they are comfortable answering any follow-up questions that their students might have.’

Thirty-seven high school teachers take the quantum course in Leiden University’s new Studio Classroom every other Tuesday from 16:30 to 20:20 in October and November.

On 28 October 2016, prospective female students explored the natural sciences at the annual Physics Ladies’ Day. This event, specially for girls in the final two years of high school, was being held for the third time.

Day the school pupils were introduced to different aspects of the Physics programme, and learned about the job prospects for graduates. They attended talks by current students of the programme, met women professionals from the world of physics, did some experimenting themselves and ended with a high tea.

Serafine & Anna (year 6)
During the lunch Serafine and Anna talked enthusiastically about the event: ‘It's well organised and we've been kept busy the whole day.' When asked what they had learned, Anna replied, 'I've always wanted to study Astronomy, but today I learned that you can combine it with Physics.' Serafine discovered that Physics is a broader programme than she imagined: 'During the speed-dating I heard that the programme also has an element of communication. That's good to know because that's also a subject I'm interested in.'

Raishna & Uma (year 5)
Several of the attendees mentioned the difference in self-confidence between boys and girls. Raishna explained: 'Today has boosted my confidence and I've become more motivated about studying Physics. You don't have to be top of the class.' Uma agreed, but was particularly keen on another aspect of the Physics Ladies' Day: 'Talking to professionals gave me a clearer idea about what you can do with a degree in Physics.'

Nalini & Kirsten (year 5)
Nalini, too, said that the Physics Ladies’ Day gave her a better idea of the job prospects: 'When I was asked to write down five professions that are possible with a degree in Physics, I only got as far as astronaut and researcher. Now I know that there are a lot more options.' Kirsten liked the personal contact with the students. 'Hearing them talk gave me a much clearer picture of what it is that they do. And you could ask them all your questions during the speed-dating.'

On Friday 28 October, LION organizes its annual Physics Ladies Day for female high school students. To mark this festive day, we put the spotlights on four female researchers, who talk about their experiences in physics.

‘As a kid, I really liked mathematics, and science in general. In school I found out that you can actually apply mathematics to science, to solve physics problems. I think it is really cool to understand things, and the equations behind it. That is why I studied Physics.

‘During my studies, I liked solid state physics the most. This subject studies how electrons move through materials, following the laws of quantum mechanics. I enjoyed the friendly atmosphere in general. Things are very informal. When I was done with my Master’s, I figured I could still have lots of fun with Physics, so I decided to pursue a PhD.

‘I went to Leiden for my PhD research because I wanted to work in an international environment. I like to get in touch with people from different places around the globe. I think this really opens up your mind. Leiden is very international and it is easy to be a foreigner here. Dutch people speak English very well and they are open to other cultures.

‘In Leiden I currently work with an electron microscope, which we named Tamagotchi. We use it to see atoms on surfaces of weird quantum materials and to understand how electrons behave inside these materials. The research I am doing in Leiden is more experimental than at my previous university in Padova, and I found out that I really enjoy working in the lab. Doing research in physics taught me how to tackle a problem in order to solve it in the best way. This skill is very useful also outside the lab.‘

Kirsten Martens, Phd student Biophysics

‘What I like most about Physics, is trying to get insight into nature by solving problems. That takes a lot of thinking, calculating and experimenting, but it gives much satisfaction once you finally reach a solution and understand the problem.

‘I arrived at my PhD position through an alternative route. When I was a kid I got interested in the natural sciences. Astronomy, cosmology and evolution of and on the Earth fascinated me so much that I chose to go study Astrophysics. In the meantime, I switched to Chemistry because those subjects attracted me more. Through my Master’s internships I finally ended up in my current job in biophysics.

‘In high school it seems like Physics and Chemistry are nothing alike, but that is far from the truth. Many processes in Physics are also studied by chemists, only from a different perspective.

‘In my research I study interactions between DNA and proteins. Using extremely sensitive devices I detect each particle individually. That way I can map out all possible interactions. Those particles are biological molecules, like proteins and DNA. This may sound like biological or medical research, but a major part of my research is optimizing and improving physical, optical and sometimes chemical techniques.’

Daniela Kraft, Associate Professor Soft Matter Physics

‘I chose to study Physics because I wanted to understand how the world works. Physics seemed to be the best option for that. It addresses the foundations of nature. Physicists describe our world in models that are only as complex as they need to be, and no more than that. I like this way of thinking and doing research. We keep it as simple as possible and look for the essence. Given the complexity of the world, I find it fascinating that we understand it so well with an often simple model.

‘During my studies, I enjoyed working together with my classmates on sums and math puzzles. I liked the feeling of a community of physicists amongst each other. I love sharing my passion for physics.

‘Now I have my own research group in Leiden. We study small particles that are large enough to see with a microscope, but small enough so they keep moving on their own. We use those particles to study complex processes in an easier way. That way we find out what the essential ingredients are to obtain a certain structure or behavior. We use those insights to build new and useful structures. In the future, we might even build tiny medical robots.’

Babette de Jong, PhD student Biophysics

‘I discovered my passion for Physics when I was actually studying something else: Biomedical Sciences. I specialized in super-resolution microscopy and developed a keen interest for the physical part of that. After my Master’s I decided to expand my experience with Physics even further and applied for a PhD in Biophysics in Leiden.

‘The best part of my research is that the questions we ask are a mix of Physics, Biology and Chemistry. The variety makes my work very interesting and satisfactory to me. It’s really fun to learn to look at a problem from different perspectives. There is always something new to learn or to try and solve. I really like the fact that our group consists of people from different countries and with different academic backgrounds. This also benefits the progress of our work.

‘We study how proteins are able to read our DNA. We are one of the first to research a specific aspect of this, which makes my project both very interesting and challenging. So my research is not only an opportunity for me to learn new things, but also to make actual new discoveries.’

On Thursday November 3rd we celebrate the official opening of two wall formulas in the Leiden city center. As of now, Snell’s law and the Lorentz force formula shine on two walls in downtown Leiden. They are part of a More info

project to display at least ten groundbreaking formulas with a Leiden origin.

Pedestrians in the Leiden city center often catch a poem painted in giant letters on a wall. This poetry is mostly written in a foreign language that few Leiden citizens understand. Still, this doesn’t pose a problem. We see the beauty in a verse without exactly knowing what it says. We feel it is art, we read the name and the time in which the poet lived and play the story in our heads.

Physicists Sense Jan van der Molen and Ivo van Vulpen realized that this principle goes just as much for physics formulas. Not everyone understands all mathematical symbols or gets the meaning of the letters, but we do feel the intrinsic beauty of the formula as a whole. A universal law, remaining undiscovered for billions of years, is now caught in a short, simple line that will hold true until the end of time. A complex natural phenomenon, like the refraction of light in a glass of water, lies wonderfully captured in only a few symbols.

Leiden has a particularly grand history in physical discoveries. That is what inspired Van der Molen and Van Vulpen to set up a project together with Stichting TEGENBEELD to paint formulas of Leiden origin on walls in its historic center. They assembled a collection of over ten groundbreaking formulas, all with a Leiden connection. The first formula was painted last year on Museum Boerhaave; the Einstein field equation.

Now, also the second and third wall formulas have been realized. People can admire the formula for the Lorentz force from Hendrik Lorentz’ old residence on the Hooigracht and Snell’s law of refraction is flaunting further down the same street. On Thursday November 3rd we celebrate the official opening with a festive event in the Kamerlingh Onnes building in Leiden. Everyone is welcome.

Snell’s law on a wall on the Hooigracht. The law describes the refraction of light when it crosses into a different medium. A straw in a glass of water seems to bend because water has a different breaking index compared to air.

The formula for the Lorentz force on the wall of Eetcafé de Hooykist on the Hooigracht. Hendrik Lorentz lived right across the street from here. The formula describes how a charged particle, for example an electron, is deflected in a magnetic field.

Piet Van Mieghem from Delft University of Technology will give an LCN2 seminar on October 28th at 16:00 in room HL214, titled 'Epidemic Spread on Networks'.

Abstract
Epidemic models are increasingly used in real-world networks to understand diffusion phenomena More info

(such as the spread of diseases, emotions, innovations, failures) or the transport of information (such as news, memes in social on-line networks and flows in functional brain networks). We will mainly focus on Susceptible-Infected-Susceptible (SIS) epidemics on networks. After a brief review of the SIS Markovian process on a graph, we will show why SIS epidemics on networks are so interesting and we will overview our recent developments.

Piet Van Mieghem
Piet Van Mieghem is professor at the Delft University of Technology with a chair in telecommunication networks and chairman of the section Network Architectures and Services (NAS) since 1998. His main research interests lie in the modelling and analysis of complex networks (such as infrastructural, biological, brain, social networks) and in new Internet-like architectures and algorithms for future communications networks.

professor at Leiden University from 1920 to 1946. Being a German citizen, he carried a special ID card. Until October 27 you can vote for this piece of heritage to be ‘Piece of the Year’. The election takes place as part of the History Month.

For Leiden University, Einstein’s ID card is a symbol for the added value of open borders in a knowledge economy. Numerous foreign scientists perform scientific research or lecture at Leiden University, and many Leiden researchers work abroad. This exchange is of major importance for the advancement of science.

Einstein enjoyed visiting Leiden, which he called ‘that delightful place on this parched earth’. Colleagues such as Hendrik Lorentz, Paul Ehrenfest, Willem de Sitter and Heike Kamerlingh Onnes helped him forward and offered new insights. Einstein often stayed at Ehrenfest’s place, which was an exceptional household filled with music and science.

Rigid materials break more easily than floppy ones. This simple observation allows to predict and control the width of cracks. Theoretical understanding of how materials break is useful in for example the production of cars or screens. Publication More info

in PNAS.

If you are unlucky enough to have broken a limb at some point in your life, did you wonder why it was the bone that broke, and not the skin? After all, the skin took the first impact. From our intuition we know that rigid materials break more easily than soft ones.

Crack
The research group of theoretical physicist Vincenzo Vitelli at Leiden University and his colleagues from the Nagel Lab have exploited this phenomenon to design materials that resist breaking. A rigid material has many bonds and generates a narrow crack in an approximately straight line (see figure 1a). A material composed of fewer bonds is softer, and produces a diffuse failure region: a crack that can be as wide as the sample size (see figure 1b). When that happens the material can resist catastrophic failure thanks to its softness. To discover this, the physicists simulated and built artificial structures—called metamaterials—with tunable numbers of bonds that break in unusual ways. They publish their findings in the journal PNAS.

Einstein
Earlier, Vitelli’s group published a paper in Nature Materials in collaboration with the Irvine Lab, on the path that a crack follows as it propagates in a curved thin layer. They discovered a remarkable parallel with Einstein’s theory of general relativity, where a ray of light is bent by the curvature of space-time. In the case of fracture, the crack path is bent by the curvature of the underlying surface (see figure 2).
‘When you have a theory on how things break, you can use it to control the properties of real materials,’ says Vitelli. ‘This is potentially useful. For example, you may want to deflect a crack from a portion of a given structure, like the centre of your glasses. Or, to prevent breaking altogether, you can design floppy metamaterials.’

Figure 1. (a)Simulations show a rigid structure with many bonds that generates a straight, narrow crack when broken. (b)In a soft structure with few bonds, the broken bonds (shown in color) are spread over a crack that can be as wide as the system size. (c)An experimental realization of a soft structure, using cellular metamaterial.

Figure 2. (a)Theoretical calculations of the crack path (shown as a black line) on a curved surface on which a crystalline monolayer is deposited. The curved spots are plotted in red and blue. The crack path is bent by these bumpy spots, just like light is bent by the curvature of space-time. (b)Experimental realization of a bent crack in curved space.

The Leidse Instrumentenmakers School (LiS) has received the Leiden erepenning—an honorary medal that is awarded each year during the celebration of the Leids Ontzet to a person or institute that has done a great service to the city of Leiden.

is a professional school for students who learn to design and create precision instruments for science and industry. The school was founded 115 years ago by Leiden physicist and Nobel laureate Heike Kamerlingh Onnes. It still has close connections to the Leiden Institute of Physics. Many graduates work at its department of fine mechanics and Prof. Peter Kes and Dr. Hans Brom are LiS board members.

Since 1997 LiS has had its own building at the Bio Science Park next to the faculty of science. It was elected the best professional school in The Netherlands in 2013 by the Selection Guide MBO. For the past several years, the number of LiS students kept increasing, making it necessary to expand the school. This expansion completed this year, with support of Stichting Utopa, the ministry of OCW, Leiden municipality and Leiden University.

The Royal Netherlands Academy of Arts and Sciences (KNAW) will host an event on Friday October 7th to highlight the Nobel prizes that will have been announced earlier that week.

On behalf of the Leiden Institute of Physics, Prof. Carlo Beenakker More info

elaborates on the Nobel prize in Physics, which was awarded to David Thouless, Michael Kosterlitz and former Lorentz Professor Duncan Haldane. Beenakker has been a KNAW member since 2002. Other speakers are Hans Clevers (Medicine), Bert Weckhuysen (Chemistry) and Maarten Asscher (Literature). This year the Nobel Prize in Chemistry went in part to Dutchman Ben Feringa from the University of Groningen.

The event is open to anyone, but registration is mandatory; see the event page.

The Nobel Prize in Physics 2016 goes to David Thouless, Michael Kosterlitz and Duncan Haldane for theoretical discoveries in the field of topological materials. These only conduct electricity on their surface and hold a big promise for future applications in More info

electronics, superconductors or even quantum computers. Haldane spent part of 2008 in Leiden; he is the 14th Lorentz Professor to win a Nobel Prize.

We are used to assign properties to materials as we experience them in our everyday life; three-dimensional objects. But if you look at extremely thin, ‘two-dimensional’ layers, you will see completely new physical phenomena at ultra-low temperatures in strong magnetic fields. A famous example is the quantum Hall effect: changing the magnetic field affects the electric conductance, but only in discrete steps.

Topological phases
Thouless and Haldane explained this effect using the mathematical field of topology. In topology, objects are classified by the number of holes. A coffee cup is therefore similar to a donut because they both contain one hole, and a sphere occupies the same category as a bowl for having zero holes. The number of holes can only be an integer, so this parameter goes up step-by-step. Thouless and Haldane linked two-dimensional layers and one-dimensional chains in a clever way to this principle, and deduced that their properties also change in steps (see figure 1).

Phase transitions
Alongside the discovery of topological phases of matter, this year’s Nobel Prize is also awarded for the discovery of topological phase transitions, made by Thouless (again) and Kosterlitz. Topological phases are not like the well-known phases solid, liquid or gas. They occur at very low temperatures in two-dimensional layers and manifest themselves as vortices of atomic magnets (see figure 2). If the temperature increases slightly, a tight pair of vortices gets separated and single vortices are created. This phase transition is a universal phenomenon for any type of material, making it an important breakthrough.

Lorentz Professor
Duncan Haldane spent part of 2008 in Leiden as Lorentz Professor. ‘It was great to have him here, we became good friends,’ says Leiden theoretical physicist Jan Zaanen. ‘He even made a big discovery during his time in Leiden: entanglement entropy for topologically ordered systems. I am delighted for him, he very much deserves this prize. He and Thouless were way ahead of their time. For the Kosterlitz-Thouless transition I have actually been expecting a Nobel prize for a long time. I’m surprised it didn’t happen earlier.’

Figure 1: Just like the number of holes can only go up step-by-step with integer numbers, the electrical conductance of 2-dimensional layers also go up in steps, as described by the quantum Hall effect. The Nobel laureates have made this link.

Bernard van Heck has won the Christiaan Huygens Prize 2016 for his Leiden PhD research on electrical circuits for quantum computers. On behalf of the KNAW, the Dutch minister of Education, Culture and Science awards this prize annually to a More info

researcher who has made an innovative contribution to science.

Majorana particles
Scientists across the globe are working hard on realizing the first quantum computer—a revolutionary machine which computes differently than conventional computers. A promising way of building one is with the use of Majorana particles, which are predicted to appear in carefully crafted superconductors. These particles possess a special property: they can 'share' a quantum state while being far apart, and 'hide' it from the environment. Furthermore, by moving the particles around each other—'braiding'—their shared state can be changed in a controllable way. These properties make Majorana particles well equipped as building blocks for a quantum computer.

Manipulate
To perform calculations, a Majorana quantum computer will have to manipulate its building blocks somehow, like a classical computer switches its transistors on and off. In this case this turns out to be difficult, however, because unlike the electrons moving in a transistor, Majorana particles have no electric charge. Van Heck, together with several colleagues, has developed a method to manipulate Majorana particles without actually touching and moving them. Importantly, the method uses standard electronic components—capacitors and inductors—in order to 'braid' the particles, and so it is practically feasible in the laboratory. Several international research groups have picked up Van Heck’s method to try and build a real-world quantum computer.

Bernard van Heck received his award, together with a cheque of 10.000 euro, from the Dutch minister of Education, Culture and Science Jet Bussemaker.

High school students Bas Teisman and Erik van Dijk have won the national final of the Junior Physics Olympiad in Leiden.

Prizes
The final consisted of three parts; a movie of an experiment, open questions and closed questions. Lucas van der Poll More info

and Erik van Dijk won the category 'movies'. Daan Timmers answered the open questions best and Richard Wols beat the rest in the closed questions.

The overall winners of the Junior Physics Olympiad are Erik van Dijk from the Eemsdeltacollege in Appingedam and Bas Teisman from the Praedinius Gymnasium in Groningen.

1,540 participants
In February of this year, 1,540 third year high school students started the first round of the Junior Physics Olympiad. Over one hundred bright young minds managed to reach the regional finals. At six different locations in The Netherlands, they performed a practical experiment and answered closed question on a test. The 31 student with the best score eventually qualified for a spot in the national final at the Leiden Institute of Physics.

Finals day
On the day of the final, the now fourth year students were offered a rich programme with alternately lectures, tests and lab tours. The morning started off with a test with open questions, followed by a lecture from experimental physics Professor Martin van Hecke on his flexible cubus. After a tour through the Kamerlingh Onnes lab and a hearty lunch, the participants wrapped their heads around a number of multiple-choice questions in the final exam. Theoretical physicist Professor Carlo Beenakker closed off the day with a lecture on ‘Weird effects of quantum’ and the prize ceremony.

The Leiden Institute of Physics announces its second annual Image Award. Master students, PhD students and postdocs from LION are asked to submit an amazing visual of their research: a microscopy image, a rendering of simulation results, a theory visualization More info

...... or just get creative!

The winner receives a first prize of 100 euro cash, eternal fame and glory, plus a prominent place in the building for their winning picture. The deadline for submission is October 31 at 23:59. Eligible participants are asked to send their image including a layman's description (Publ. 28-09-2016 16:32

This year, the Junior Physics Olympiad final takes place on September 29 at Leiden University. Thirty-one high school students have made it into the national final.

1,540 participants
In February of this year, 1,540 third year high school students started the first More info

round of the Junior Physics Olympiad. Over one hundred bright young minds managed to reach the regional finals. At six different locations in The Netherlands, they performed a practical experiment and answered closed question on a test. The 31 student with the best score eventually qualified for a spot in the national final at the Leiden Institute of Physics.

Finals day
On the day of the final, the now fourth year students are offered a rich programme with alternately lectures, tests and lab tours. The morning starts off with a test with open questions, followed by a lecture from experimental physics Professor Martin van Hecke on his flexible cubus. After a tour through the Kamerlingh Onnes lab and a hearty lunch, the participants wrap their heads around a number of multiple-choice questions in the final exam. Theoretical physicist Professor Carlo Beenakker closes off the day with a lecture on ‘Weird effects of quantum’ and the prize ceremony.

Leiden physicist Rudolf Tromp has been awarded the 2017 Distinguished Lectureship on the Applications of Physics by the American Physical Society. As part of his lectureship, Tromp will deliver a lecture series on his career as a physicist in industry. More info

Since 1983 he has worked as a scientific researcher at IBM T.J. Watson Research Labs in Yorktown Heights, New York, in areas of both basic and applied science. As of 2006 he is also a professor at Leiden University. In his lectures for young scientists, Tromp will elaborate on the possibilities and opportunities of working in industry as a physicist.

Synthetic fuel is cleaner than natural oil, but its production process needs to be more efficient. Now for the first time, physicists have directly observed the molecules produced in the chemical process. This paves the way for designing more efficient More info

To date, natural oil still serves as our primary source of fuel, even though a much cleaner alternative exists in the form of synthetic fuel. This contains much less sulfur and doesn’t require oil as the starting material. At the moment, only five percent of the world production of diesel fuel makes use of this process, because it is cheaper for companies to use more polluting substances. If researchers have a better understanding of the production process of synthetic fuel, the balance could tip the other way.

Observation
Now, physicists from Leiden University have seen for the first time how this chemical process unfolds in the early stages. It was already known that the necessary chemical reaction between carbon monoxide and hydrogen takes place on the surface of small cobalt particles. These serve as the catalyst for the reaction. It is however very difficult to verify the exact working mechanism in experiments. Researchers have to deal with pressures of several atmospheres and temperatures of several hundred degrees Celsius. These are far from ideal circumstances to observe molecules. Group leader Joost Frenken and his team developed a special type of Scanning Tunneling Microscope—a so-called Reactor-STM—to bypass this problem.

Parking lot
To their surprise, they observed that in the first stages of the process, the surface covers itself up progressively in a single layer of hydrocarbon molecules with a highly ordered regular pattern. The molecules accumulate on the cobalt surface all with the same, surprisingly long length. The Leiden physicists were able to explain these findings with a simple theory, in which the catalyst constructs the molecules step-by-step at the atomic steps on the cobalt surface. Most molecules spend some time on the surface and then evaporate, but the longer ones stick more strongly and fill the surface. The most efficient way to do that is to fill the surface with a regular pattern, similar to cars in a parking lot.

Designer catalyst
Currently, catalysts are being developed mostly by trial and error. With the new discovery, first author Violeta Navarro and her colleagues pave the way for future generations of genuine ‘designer’ catalysts, with fully optimized efficiency and selectivity for the desired products. Frenken: ‘The ultimate goal is that of true “designer catalysts”. We’re absolutely not there yet, but understanding the early stages of synthetic fuel production forms an essential component of unraveling the entire, complex reaction mechanism. We have introduced a new way of looking at an active catalyst with the ultimate resolution.’
This research was supported by a Veni grant from STW.

Top: Artist impression of the reactants carbon monoxide and hydrogen and the produced hydrocarbon molecules of different lengths on the cobalt catalyst (carbon atoms are represented in green, oxygen atoms in red and hydrogen atoms in blue).
Bottom left: Topographic image of a region (3840 nm2) of the surface of a cobalt catalyst during reaction, obtained with a scanning tunneling microscope. The height in the image is represented in a color scale where the darker colors are lower than the lighter ones, and the total height is 1.4 nm. The image was taken after 40 minutes of reaction at 221 °C and a pressure of 4 atmospheres in a mixture of the gases carbon monoxide, hydrogen and argon in the ratio 1:2:2 respectively. The cobalt surface is covered by a stripped pattern which results from the organization of the molecules produced during the reaction, that align next to each other in a regular pattern, similar to cars in a parking lot. The magnifying glass shows an impression of how the molecules are organized within the stripes.
Bottom right: Graphic representation of the concentration of the molecules produced during the reaction on the cobalt catalyst surface as a function of the reaction time, depending on their length. All lengths are produced during the reaction, but the shorter molecules are very volatile and leave the surface fast. Longer molecules reach higher concentrations on the surface of the catalysts. The molecules with 15 carbon atoms are the first ones that reach a concentration on the surface which is high enough to organize themselves in a striped regular pattern.

For the first time, physicists have visualized the ‘melting’ of electrons inside a special class of insulators. This process allows electrons to move freely and turns the insulator into a metal and possibly even a superconductor. Publication on September 19th More info

Electronic liquid
Some materials carry an electrical current more easily than others. Metals are for example world class conductors. Inside them, the electrons form an electronic liquid that flows through the atomic lattice. In specific insulators on the other hand, electrons are stuck to their place in the lattice; the electronic liquid is frozen (see image below). In these so-called Mott insulators, you can replace some atoms with different ones. Physicists call this ‘doping’. It is known that doping leads to a melting of the frozen electronic liquid, but nobody knows how this process works.

Puddles
Now, Leiden physicist Milan Allan together with lead authors Irene Battisti and Koen Bastiaans have, for the first time, visualized this melting process in a family of materials called iridates. They discovered that the melting process is very inhomogeneous, with puddles forming in between frozen areas. These puddles are only a few nanometers in size (see image below). The research group, in collaboration with theoretical physicist Jan Zaanen, publishes their results in Nature Physics.

Superconductivity
Apart from getting insight in a very fundamental process, the discovery also shines light on the mystery of superconductivity—a phenomenon where electrons move without resistance. Superconductivity is important because it allows transportation of electricity with zero energy loss. ‘We came to believe that this kind of melting is a universal prerequisite of superconductivity,’ says Allan. ‘If we would manage to melt the electronic liquid in all parts of the sample, it would likely become a new superconductor.’

The melting of electrons. In the blue areas, the electrons (red dots) are stuck to the atoms in the lattice (green circles), meaning that there is no current. In the red areas, dopant atoms (black circles) are added, giving the electrons room to move and making them behave like a liquid. The researchers expect that once the whole area is molten, the material is a high-temperature superconductor.
Left: Actual measurement. Right: Illustration of concept.

PhD student Rik Mom has been awarded the Michel Cantarel grant by the French Vacuum Society (SFV). At the European Conference on Surface Science he gave a prize-winning talk on the subject of his PhD research, making him More info

one of five award winners. During his PhD research, Mom images catalysts at the atomic scale while they are operating in chemical reactions. Performing such studies under realistic conditions is essential. ‘Atomic scale studies on catalysts are often done in ultrahigh-vacuum and at low temperatures,’ he says. ‘This is not representative of real-life processes, which happen under pressures of 1 to 100 bar and at temperatures between 100 and 400 °C.’

To obtain results that are useful for real applications, Mom studies nanoparticle catalysts in chemical reactions at 1 bar and 250 °C, as he explained in his talk. In an effort to also improve the realism in the model catalyst that is used in the studies, he published a paper in The Journal of Physical Chemistry C on his successful attempt to control the preparation of complex, realistic model catalysts.

Koenraad Schalm and his research group publish an article on non-Fermi liquids in the current issue of Physical Review B. Their publication has been selected as Editor’s suggestion.

Metals are good conductors: the outermost electrons can nearly effortlessly skip More info

from atom to atom. The theory behind this—electronic band structure based on Fermi liquid theory developed in the 1950’s—explains why a metal conducts electricity and other non-metallic materials do not. This theory lies at the heart of modern semi-conductor electronics.

In exotic materials, however, electrons interact much stronger than usual and our theoretical understanding fails. High-temperature superconducting copper oxides form a good example. Almost by definition, these materials behave as a non-Fermi liquid, meaning they don’t obey Fermi liquid theory. Schalm and his colleagues describe their research on this topic in a popular summary on the Editor’s suggestion page of Physical Review B.

Proteins organize themselves around our body cells through a self-induced force. They indent the cell membrane and thereby roll towards each other. This discovery provides new insights in processes like nutritional uptake and brain signaling, as well as in diseases More info

How do you study something that is too small to be seen? This is what Leiden physicists wondered when they wanted to study how proteins organize themselves within our body. Proteins are responsible for important processes in the body and are as tiny as a molecule. The answer appears surprisingly simple: by developing a model system on a larger length scale and ensuring that this model is consistent with the smaller reality. Still, this is easier said than done.

Deformations
Group leader Daniela Kraft and first author Casper van der Wel describe their scaled-up model system in Nature’s Scientific Reports. They discovered that their model proteins organize themselves by deforming the protection layer in and around cells – the cell membrane. Until now, experiments had only been able to convincingly measure protein interactions through conventional forces like the electrostatic force or gravity. The membrane-mediated force is based on minimizing the energetic cost for the deformation: when proteins attach to a cell and deform the cell membrane, it is often energetically more favorable for them to be close to each other. And because nature always chooses the option of the least energy, this is indeed what happens (see figure 1). By indenting the protection layer, they attract each other.

Scaled-up model
Kraft and her team made their discovery with scaled-up versions of proteins. They used spherical colloids—particles a micrometer in size that behave like the proteins at the nano level despite their relatively large diameter. Kraft: ‘The discoveries from our model system also hold for the much smaller proteins, because our results strongly agree with theoretical simulations that are independent of the length scale. In fact, it holds for any membrane-deforming object as long as it is larger than the membrane thickness.’

Binding energy
The biophysicists know that the measured deformation is indeed the driving force behind the attraction because they discovered that the spherical particles can only be in two states: either the particles deform the cell membrane or they don’t. And only in the case of deformations, the model proteins are attracted towards each other. If the binding energy between the model protein and the cell is larger than the energy it takes to deform the protection layer, the layer instantly wraps around the protein. If not, then there is no deformation.

Important proteins
The Leiden results give a unique insight into the organization of one of the most important building blocks in our body. They help cells to take up nutrients and collaborate to regulate brain signaling. On the other hand, incorrect protein organization can have severe consequences. For example, protein accumulations are one of the causes of Alzheimer’s disease. With the new quantitative insights, scientists may also understand how these processes come about.

Figure 1. Left: Model proteins (green) attach to a cell and indent the cell membrane. Right: It is often energetically more favorable if the proteins are closer to each other. And because nature always chooses the option that costs the least energy, this is indeed what happens.

Two model proteins (green) move toward each other by indenting the cell (purple). This way, the deformation acts as an attractive force. Leiden physicists have now for the first time measured this force and made it visible.

Physicists have manipulated light with large artificial atoms, so-called quantum dots. Before, this has only been done so well with actual atoms. It is an important step towards light-based quantum technology. Publication on August 30th in Nature Communications.

a presentation, when you point a laser pointer at the screen, an immense amount of light particles race through the air at a billion kilometers per hour. They don’t travel in a continuous flow, but in packages containing varying numbers of particles. Sometimes as much as four so-called photons pass by, and other times none at all. You won’t notice this during your presentation, but for light-based quantum technology it is crucial that scientists have control over the number of photons per package.

Quantum dots
In theory you can manipulate photons with real individual atoms, but because of their small size it is extremely hard to work with them. Now, Leiden physicists have discovered that the same principle goes for large artificial atoms—so-called quantum dots—that are much easier to handle. In fact, they managed to filter light beams with one photon per package out of a laser. ‘Another big advantage of quantum dots is that the system already works within nanoseconds,’ says first author Henk Snijders. ‘With atomic systems you need microseconds, so a thousand times longer. This way, we can manipulate photons much faster.’

Quantum cryptography
The ultimate goal for the research group led by Prof. Dirk Bouwmeester is to entangle many photons using quantum dots. This is essential for example in techniques like quantum cryptography. Snijders: ‘This research shows that we are already able to manipulate individual photons with our system. And the beauty is that in principle we don’t need large experimental setups. We can just integrate our quantum dots in small microchips.’

Catalyst research aims to make gasoline less polluting. It turns out that during experiments it is actually necessary to protect catalysts from air itself. Publication in The Journal of Physical Chemistry C on August 26.

catalysts that nowadays gasoline is much less polluting than earlier. Crude oil contains the harmful substance sulfur, which refineries filter out in the process of turning oil into gasoline. They add hydrogen and for example the catalyst NiMoS2. The hydrogen removes a sulfur component of NiMoS2, giving the catalyst room to collect the sulfur from the oil.

Clean sample
To further improve the process, scientists research substances like NiMoS2. A small adaptation in the chemical composition could make it a more efficient catalyst. In such experiments it is important to know how to keep the studied sample free from external influences.

Harmful air
A group of physicists led by Joost Frenken (Leiden University) and Patricia Kooyman (University of Cape Town) together with TU Eindhoven have now shown that exposure to air is very harmful. ‘Oxygen molecules from the air oxidize the NiMoS2 catalyst particles, so that further studying the sample essentially produces no relevant information,’ says first author Marien Bremmer. ‘We noticed that the oxidation occurred extremely fast at first, but slowed down in the long-term. This indicates the formation of a shielding oxide ring.’

Publication
The research group used a high-resolution transmission electron microscope (HRTEM) to see that after 24 hours already twenty percent of each NiMoS2 particle is covered with oxygen. They describe the study in The Journal of Physical Chemistry C.

After 24 hours, a clean NiMoS2 particle (yellow-red) is already 20% covered with oxygen (blue spheres). In the subsequent period this process settles down, but a month later the coverage still has significantly increased.

The LCN2 community is proud to present the Leiden Networks Day: a one-day symposium open to all researchers interested in networks, from Leiden, the Netherlands and beyond. The event will feature a number of excellent international speakers and is free More info

Nature publishes an article on a paradoxical discovery in superconductivity. Leiden physicist Jan Zaanen writes a News & Views article about this in the same issue of August 19th.

Superconductivity is a bizarre but useful physical phenomenon. By More info

cooling a material down to below a critical temperature, its electrical resistance suddenly disappears completely. That way, you can easily send electricity through a wire without any loss of energy. This comes in very handy for example in windmills or MRI scanners.

News & Views
The cooling however poses a problem, which finally does cost energy. That is why physicists are on the hunt for a material with superconductive behavior at not-too-low temperatures. In a News & Views article in Nature, Leiden physicist Jan Zaanen describes how a new discovery leads to an interesting paradox.

Pairs
The discovery, made by Ivan Božović from Yale University, has to do with copper oxides. In principle these don’t conduct any current. Their electrons have too strong of an interaction and retain each other, like cars in a traffic jam. But taking away some electrons gives the rest some room to manoeuver. They do this in pairs. These electron pairs can actually move so well that superconductivity occurs, even though the temperature is relatively high.

Paradox
By removing too many electrons, a surplus of empty spots arises. Electrons now have difficulty finding each other to form pairs, and the value for the critical temperature drops. Everything seems to indicate that the famous Bardeen-Cooper-Schrieffer (BCS) theory from 1957 is applicable in this case. This theory describes quantum physics of conventional superconductors very precisely. BCS counter-intuitively predicts that all electrons take part in superconductivity, even if the critical temperature value is extremely low. However, Božović sees the number of participating electrons diminish proportionally to the critical temperature value. Like Zaanen describes, this as an apparent paradox which cannot be explained with our current understanding of quantum physics.

On an apparently normal cube a pattern of hollows and bulges appears when the cube is compressed. Physicists from Leiden University and FOM Institute AMOLF together with colleagues from Tel Aviv University have developed a method to design such three-dimensional More info

structures and to construct these using simple building blocks. This paves the way for the use of 'machine materials' in, for example, prostheses and wearable technology. The researchers will publish their findings on 28 July in Nature.

Normally, atoms and molecules determine the properties of the materials they form. However that is different for 'metamaterials' designed by humans. "In the case of metamaterials, the spatial structure determines the material's behavior," explains group leader Martin van Hecke. "For example, a pattern of holes in a sheet of material gives rise to a mechanical response that is completely different than in the same material without holes. We also wanted to investigate this phenomenon for a three-dimensional pattern of holes."

3D puzzle
Van Hecke and his colleagues designed a cube-shaped, flexible building block with a hole in it. If pressure is applied to such a block then some of the sides cave in, whereas others bulge out. By stacking several of these building blocks researchers could make three-dimensional structures. Van Hecke: "The orientation of the blocks in the metamaterial is important. Under pressure, all of the hollow and bulging sides must fit exactly together. Most of the stacks are 'frustrated': somewhere within two hollows or bulges meet. However a large number of fitting solutions for this three-dimensional puzzle were found."

Patterns
Van Hecke's colleagues at Tel Aviv University calculated the number of possible, non-frustrated stacks for different cubes of building blocks. "For one cube of 14x14x14 building blocks that is a number with no less than 65 figures," says Van Hecke. "For each possible stack the deformation within the cube results in a specific pattern on the sides of the cube. By smartly combining the building blocks we can program the material such that every desired pattern appears on the sides of a compressed cube. Surprisingly such a cube can also be used to analyse patterns. If we press it against a pattern of hollows and bulges then we measure a force that is dependent on the pattern."

Machine materials
Although Van Hecke's research is fundamental in nature there are applications on the horizon. "This type of programmable 'machine materials' could be ideal for prostheses or wearable technology in which a close fit with the body is important," says Van Hecke. "If we can make the building blocks more complex or produce these from other materials then the possibilities really are endless."

Image Caption
To demonstrate that any pattern can be produced on a cube's surface, the researchers developed a cube of 10x10x10 blocks on which a smiley appears when the cube is compressed.
Credits: Corentin Coulais

Physicists have studied the astrophysical neutrino signal as reported by the IceCube collaboration from a different angle with their ANTARES detector. The Milky Way centre was an obvious prime suspect to be a source, but this hypothesis is now only More info

Gotta catch ‘em all! Physicists are always in the hunt for any kind of particle raining down from the sky. Amongst them are neutrinos—one of the hardest to catch. These ultralight particles are so difficult to detect because they penetrate through anything, including detectors. This also means that they are extremely interesting for scientists, because they travel from the inside of space objects directly to Earth, without getting deflected along the way. And with that, they keep a bunch of information safely stored inside them.

IceCube
To catch them, scientists need massive detectors made of several cubic kilometers of ice or water, like IceCube on Antarctica or ANTARES in the Mediterranean Sea. IceCube has recently reported many detected neutrinos, with a higher number coming from the Southern sky. The centre of the Mily Way is located there, so our Galaxy’s core was an obvious prime suspect to be responsible for a good part of this neutrino influx. However, the signal events in IceCube have a limited resolution, so it remained unclear where the mysterious signal comes from.

ANTARES
Now an international team of physicists, including Leiden University’s Dorothea Samtleben, have used the ANTARES detector to look at the signal at high resolution from a better angle. They show that under certain plausible assumptions on the neutrino flux properties only two of the events detected by IceCube could originate from the Milky Way. ANTARES will now continue with a newly developed reconstruction method to also probe even higher energetic neutrino fluxes from the Milky Way as cause.

Suspects
‘We are assuming that the so far detected astrophysical neutrinos come from sources with violent “explosions”,’ says Samtleben. ‘We don't know whether the detected neutrinos come from our own galaxy or from outside, and so far also no significant correlation of the neutrino directions could be found with any other known astrophysical source’.

The ANTARES team publishes their results in Physics Letters B on 10 September, but the article is already accessible online.

A new NMR microscope gives researchers an improved instrument to study fundamental physical processes. It also offers new possibilities for medical science, for example to better study proteins in Alzheimer patients’ brains. Publication as Editors' Suggestion in Physical More info

Review Applied.

If you get a knee injury, physicians use an MRI machine to look right through the skin and see what exactly is the problem. For this trick, doctors make use of the fact that our body’s atomic nuclei are electrically charged and spin around their axis. Just like small electromagnets they induce their own magnetic field. By placing the knee in a uniform magnetic field, the nuclei line up with their axis pointing in the same direction. The MRI machine then sends a specific type of radio waves through the knee, causing some axes to flip. After turning off this signal, those nuclei flip back after some time, under excitation of a small radio wave. Those waves give away the atoms’ location, and provide physicians with an accurate image of the knee.

NMR
MRI is the medical application of Nuclear Magnetic Resonance (NMR), which is based on the same principle and was invented by physicists to conduct fundamental research on materials. One of the things they study with NMR is the so-called relaxation time. This is the time scale at which the nuclei flip back and it gives a lot of information about a material’s properties.

Microscope
To study materials on the smallest of scales as well, physicists go one step further and develop NMR microscopes, with which they study the mechanics behind physical processes at the level of a group of atoms. Now Leiden PhD students Jelmer Wagenaar and Arthur de Haan have built an NMR microscope, together with principal investigator Tjerk Oosterkamp, that operates at a record temperature of 42 milliKelvin—close to absolute zero. In their article in Physical Review Applied they prove it works by measuring the relaxation time of copper. They achieved a thousand times higher sensitivity than existing NMR microscopes—also a world record.

Alzheimer
With their microscope, they give physicists an instrument to conduct fundamental research on many physical phenomena, like systems displaying strange behavior in extreme cold. And like NMR eventually led to MRI machines in hospitals, NMR microscopes have great potential too. Wagenaar: ‘One example is that you might be able to use our technique to study Alzheimer patients’ brains at the molecular level, in order to find out how iron is locked up in proteins.’

NMR microscope, consisting of a thin wire and a small magnetic ball (fake colour purple). The purple ball induces a uniform magnetic field, so that the surrounding atomic nuclei all line up with their axis pointing in the same direction. The researchers send radio waves through their sample, causing some nuclei to flip the other way, and measure how long it takes before they flip back again.

Physicist Scott Waitukaitis receives an NWO Veni grant to research the Leidenfrost effect for squishy materials. This effect is well-known for dancing water droplets in a frying pan.

Have you ever spilled water on a hot frying pan? You will see More info

the water droplets do a funny dance, floating around on a thin layer of vapor. This is known as the Leidenfrost effect. And as silly as this effect may seem, it is used in serious applications. It is for example essential in nuclear reactors, where the Leidenfrost effect makes sure that the cooling water doesn’t directly touch the smoldering hot nuclear fuel rod. In case of direct contact, the water would immediately vaporize in a giant explosion.

Leiden physicist Scott Waitukaitis is interested in the fundamental science behind this effect, especially for squishy materials. NWO has awarded him a Veni grant to perform experimental research on the subject. 'We all know that water floats on a hot surface, and the same goes for stiff solids like dry ice,’ he says. ‘But squishy materials bounce up and down (see video below, edit.). No-one has studied this phenomenon and with this Veni grant I’m going to conduct experiments using high-speed cameras to look at the height of the bounces, the temperature dependence, the sounds, the amount of water is vaporized per bounce, etcetera.’

In the video below you see hydrogel spheres which are fully soaked with water. Waitukaitis is going to develop formulas to describe the exact behavior of the balls.

Stars orbit black holes while jumping up and down. This is the prediction of a theoretical model developed by Leiden physicist Satish Kumar Saravanan, based on Einstein’s theory of relativity. He defends his PhD thesis on July 7th.

a few certainties in life. One of them is that the sun always rises in the East and sets in the West. However, this won’t keep happening in 24-hour cycles forever. Because of friction, the Earth slightly slows its rotation every year. The law of preservation of angular momentum dictates that consequently the Moon must move away from the Earth. Think of a ballerina: if she spreads her arms, her body will rotate slower.

Einstein
We can describe the mechanics behind the Earth-Moon system with ‘Newtonian’ physics. If we move into more extreme territory, such as supermassive black holes with the mass of a million suns, we need Einstein’s theory of relativity. Heavenly bodies orbiting around them reach velocities close to the speed of light and the large black hole mass curves space dramatically. This leads to ‘relativistic’ effects which Newtonian physics cannot describe. Physicist Satish Kumar Saravanan has now for the first time done calculations for spinning bodies orbiting a black hole, starting from a full relativistic approach instead of using Newtonian formulas. He defends his PhD thesis in front of his supervisor Jan-Willem van Holten on July 7th in Leiden.

Black hole orbits
In his theoretical study, Satish discovered different types of orbits of a rotating star around a non-rotating black hole. In a scenario where the star spins around its axis at a varying pace, its orbit will keep changing (see figure 1). We already see this in Mercury’s orbit around the Sun. Now, Satish has discovered that this irregular behavior in orbits is due to the relativistic effects of the fast rotating orbiting body.
In a more realistic scenario, the star has a spin precession, just like Earth; its tilted axis precesses like a spinning top toy. As a consequence, the star will move up and down during its orbit (see figure 2). The same actually happens with the Earth, but its velocity and the Sun’s mass are so low that this effect is negligible. In 1916, Willem de Sitter already predicted this for the Moon, and Satish has now found the same effect in systems with a rotating star around a black hole.

eLISA
Satish’ formulas will come in handy in 2034, when the eLISA space instrument will launch into an orbit around the Sun and measure gravitational waves from black hole systems. From these, researchers will deduce the orbits of objects around them, by using theoretical models like the one predicted by Satish. As black holes cannot be directly observed, this is a very useful tool for scientists.

Figure 1: In a scenario where the star spins around its axis at a varying pace, its orbit will keep changing.
Figure 2: In a more realistic scenario, the star has a spin precession, just like Earth; its tilted axis precesses like a spinning top toy. As a consequence, the star will move up and down during its orbit.

Learning about the newest communication technology in China and solving a business case at telecom giant Huawei. Four Leiden science students get to know China's Silicon Valley.

Honours students

The four honours students and fifteen other students from Malawi, the Netherlands, Spain More info

and Tunisia will be following the educational programme 'Seeds for the Future' in China from 8 to 23 July. They were selected on the basis of their excellent performance, their letter of motivation and an interview at Huawei. The Leiden students are Lars Suanet (IT), Lars Koekenbier (Maths and Physics), Joris Carmiggelt and Erik de Vos (both Physics).

Historic locations and the latest technology

They will be spending the first week in Beijing, where they will have lessons in Chinese language and culture and visit historic locations, such as the Great Wall of China. To give them greater insight into the relations between China and the Netherlands, they will be talking with staff from the Dutch Embassy. The programme for the second week includes a training course on the latest trends in ICT in Shenzhen—one of the fastest growing technology cities in China and known as the Chinese Silicon Valley. It is also where the headquarters of Huawai is located. The students will study a business case on the most up-to-date telecoms developments and will present their solution when they are back in the Netherlands. ‘We are asking a lot of the students, but that's what they want, too,' says Ka yee Lam from Huawei. ‘It's exactly this kind of practical case that gives them a better understanding of developments in the telecom and IT sector.'

Getting to know China

Why is Huawei offering this broad orientation visit? Lam: ‘The ICT sector is growing very rapidly, but the number of ICT professionals isn't increasing at the same rate. We want to make young people throughout the world more aware of the developments and opportunities in this sector. Not only that, Huawei is an ambassador for Chinese business life. We want to remove the stereotypes about the country and let students experience the real China.' IT student Lars Suanet comments: ‘Huawei is creating goodwill for the company. We are still students for the time being, but later we will all be working in the field of science or in the business sector and then we may well come across the company again.'

Be modest

In preparation for their adventure in China the Leiden students followed a lesson in Chinese culture at the end of June. Yun Tian, China consultant at the Faculty of Science, gave them some tips on how to avoid misunderstandings. For example, in China you should never finish all the food on your place, she warned them. If you do, your host or hostess will keep on putting more food on your plate until you leave some. She also advised the students to be modest and to avoid confrontations. 'In China you are not expected to brag about your achievements. It's better to stress that you are there to learn.'

Physics students are similar to Chinese students

The opportunity to learn is exactly what the four Leiden students want. They're not afraid of a culture shock. Physics student Joris Carmiggelt commented: ‘Physics students are generally very modest and try to avoid conflicts, just as the Chinese do. In that respect we have some similarities.' He is very curious about what it will be like in China in reality. ‘I often speak to Chinese students at the faculty here, but they are busy with their lives here and don't really say much about what it's like in China. Not only that, it's a great opportunity for a student to take a look inside a major international corporation.'

Exploring the job market

The same applies to the other students. Physics student Erik de Vos: ‘Budapest is as far east as I have ever been and I think it's great to be going to a completely different country from the Netherlands. A telecoms company like Huawei also employs physicists so it's a good opportunity to explore the telecoms market.'

Huawei is deeply embedded in the Netherlands

Lars Koekenbier, a student of maths and physics, remarked: 'They have a very different culture so we will all gain some very different impressions. The whole stay, including travel and accommodation, are all arranged for us. Why wouldn't you want to go?' Lars Suanet said: ‘Huawei is one of the biggest telecom giants in the world. Before this I didn't know that they do so much in the Netherlands. They seem to be deeply embedded in our communications systems and were even involved in installing the KPN network, for example.’ He is also looking forward to making contact with students from other parts of the world. 'I'm treating this trip as one big networking event.'

Two Leiden physicists took part in the Lindau conference in Germany from 26 June to 1 July. They met Nobel prize winners from their field.

The Lindau conference is an annual event where dozens of Nobel prize winners meet the next More info

generation of leading scientists from all parts of the world. From Leiden, postdoc Johannes Jobst and Adrian Hamers, who recently obtained his PhD, were present.

Strict selection
Jobst and Hamers were both nominated for the meeting by their professor. A selection committee from the KNAW then scrutinised their CV, publications and letter of motivation, before reaching a decision on who would be invited to the meeting. In total, only five Dutch scientists were invited.

Electron microscopy
Jobst is conducting research in Leiden on low-energy electron microscopy. He describes being invited to the Lindau conference as a big honour. 'Normally I go to lectures where new discoveries in my field are discussed. These Nobel prize winners talked about relatively old science, and very diverse subjects. It has really broadened my knowledge. And the lectures were exceptionally good.'

Atom bomb
Jobst was particularly impressed with the almost 90-year-old Roy Glauber, who won the Nobel prize in 2005. 'He explained his contribution to the Manhattan project, the development of the first atom bomb, and showed us some of his personal photos on which you could see Robert Oppenheimer, Albert Einstein and other famous people.’

Originality
Hamers recently obtained his PhD based on a study of the orbits of celestial bodies. He was most looking forward to the lecture by Gerard ’t Hooft, who won the Nobel prize for physics in 1999. Hamers: ‘He is a particular inspiration for me because he applied so much originality and inventiveness in physics. You always need new ideas in our field. That's the only way to discover what the implications of existing laws are in the natural world.'

Professors in science communication Bas Haring and Ionica Smeets organize a summer course titled ‘Big Science’ this week, for around fifty participants of all ages and backgrounds. On the afternoon of Thursday July 30th, the course will More info

visit the Leiden Institute of Physics.

PhD students Bart Clauwens and Kate Sokolova will give lectures on respectively cosmology and biophysics. Afterwards, the participants will enjoy a tour through the Kamerlingh Onnes laboratory, where they will be introduced to the experimental setups of the research groups of Sense Jan van der Molen, Tjerk Oosterkamp, Milan Allan and Dirk Bouwmeester. Hosts of the lab tour are PhD students Daniel Geelen, Jelmer Wagenaar, Koen Bastiaans and Frank Buters.

The course ‘Big Science’ teaches anyone that is interested in science about the basics of a multitude of disciplines, like chaos theory, probability theory, quantum mechanics and evolution.

From July 10 to July 20, the Leiden Institute of Physics organizes its annual summer school, this year titled ‘Modern Physics at all Scales’. Second and third year bachelor students from all over the world will come to Leiden to More info

participate in a program filled with talks, lab tours and cultural events.

A total of 31 students has been accepted to join the summer school out of a pool of talented students from countries in and outside Europe, including Australia, Nepal and Turkey. Collaborations between Leiden Physics and universities in Brazil and Ukraine also bring students from those parts of the world.

The participants will enjoy talks from most of the Physics principal investigators in Leiden and join in ‘hands on’ lab tours in all of the Huygens and Kamerlingh Onnes laboratories. The cultural program consists of a pub quiz, a beach barbecue, museum visits and a bicycle tour. Click here for the full program.

Leiden biophysicist Stefano Coppola has received the prestigious AXA Research Fund postdoctoral fellowship. With this grant he can work for two years on a project to research the role of mechanical factors in the development of pancreatic cancer.

the deadliest of all solid tumors. Doctors usually perform the diagnosis when the cancer has already metastasized. Because of the complexity and variety of metastatic pancreatic cancer, it is virtually impossible to cure. The best hope for reducing the mortality rate of pancreatic cancer lies in the diagnosis and treatment in the early stages.

Connection
There is persuasive evidence from other types of tumors that mechanical cues are connected with cancer progression. These could be mechanical pressure, stress or strain. On the one hand, a tumor causes its environment to become stiffer. On the other hand, a tumor changes how cells sense the mechanical cues, like stiffness, that are present.

PanIN
During his fellowship, Coppola will investigate the processes that drive the initiation and progression of a specific type of mechanical distortion in the pancreas, called PanIN. It is well known that significant mechanical stress is a hallmark feature of pancreatic cancer. However, the way in which cells convert mechanical stimuli into biomolecular activity has so far been unexplored for pancreatic cancer. Coppola hypothesizes that mechanical cues represent a novel early and label-free diagnostic biomarker to explain how pancreatic cancer progresses.

Force
Together with principal investigator Thomas Schmidt, Coppola will study the mechanical phenotypes of PanIN-damaged cells compared to normal pancreatic cells, by applying external forces on them and by observing forces exerted by cells on their surroundings. Understanding the role of force and mechanics in a developing tumor might eventually lead to novel molecular targeted therapies to predict and temper risks in early pancreatic cancer.

Leiden theoretical physicists have proven that not only the genetic information in DNA determines who we are, but also DNA’s mechanics. Helmut Schiessel and his group simulated many DNA sequences and found a correlation between mechanical cues and the way More info

Watson and Crick
When James Watson and Francis Crick identified the structure of DNA molecules in 1953, they revealed the way in which DNA contains the information that determines who we are. The sequence of the letters G, A, T and C in the famous double helix determines what proteins are made within our body. If you have brown eyes for example, this is because a series of letters in your DNA encodes for proteins that build brown eyes. Still, each cell in our body contains the exact same letter sequence, and yet every organ behaves differently. How is this possible?

Mechanical cues
Since the mid 80s it has been hypothesized that there is a second layer of information on top of the genetic code: DNA’s mechanical properties. Each of our cells contains two meters of DNA molecules, so these molecules need to be wrapped up tightly to fit inside a single cell. The way in which DNA is folded, determines how the letters are read out, and therefore which proteins are actually made. In each organ, only relevant parts of the genetic information are read, based on how the DNA is folded. The theory goes that mechanical cues within the DNA structures determine how DNA prefers to fold.

Simulation
Now for the first time, Leiden physicist Helmut Schiessel and his research group provide strong evidence that this second layer of information indeed exists. With their computer code they have simulated the folding of DNA strands with randomly assigned mechanical cues. It turns out that these cues indeed determine how the DNA molecule is folded into so-called nucleosomes. Schiessel found correlations between the mechanics and the actual folding structure in the genome of two organisms—baker’s yeast and fission yeast. With this finding we know that evolutionary changes in DNA—mutations—can have two very different effects: the letter sequence encoding for a specific protein can change or the mechanics of the DNA structure can change, resulting in a different packaging and accessibility of the DNA and therefore a different frequency of production of that protein.

The painting of the Einstein field equation on the front wall of Museum Boerhaave is now upgraded with an illustration. Last November, the equation was officially launched as the first of ten formulas that will be painted on More info

walls around the Leiden city center, alongside the already existing wall poems.

On the illustration, we see light from a distant object traveling to Earth. Along the way it is bent by the curved spacetime around the Sun. This is exactly what the Einstein field equation tells us; mass and energy manifest themselves through gravity by curving spacetime in its vicinity. In 1919, the corresponding general theory of relativity was tested using the Sun as a laboratory, in a similar setup as the illustration depicts. During a solar eclipse, Arthur Eddington measured the position of the Hyades star cluster, which was in the same place on the sky at that moment. If massive objects bend spacetime, and starlight consequently follows a curved path around them, then the Hyades cluster should be visible at a slightly different position, as its light passes through spacetime close to the Sun. And indeed, this is what Eddington measured, producing the first experimental evidence of Einstein’s theory of general relativity and his field equations.

Leiden physicists use a new method to measure so-called surface plasmons. Researching these particles could lead to new light-based technology, including faster internet. Publication in ACS Photonics.

Optical fiber
When people are watching the new Game of Thrones episode, they usually don’t More info

think about the billions of flashes that race through a worldwide optical fiber network every second to get the show on the screen. Meanwhile those flashes also encounter numerous nodes, for example in the router at home. Routers however can’t do anything with fiberglass; they work with ‘regular’ metal wires. The flashes of light are therefore transformed into an electrical signal. Afterwards, the router sends the signal forward again in the form of flashes. All this tinkering around takes a lot of time and it doesn’t make the stream any faster.

Plasmons
Downloading would go much faster if routers could use flashes instead of an electrical signal. And indeed it is theoretically possible to send light through metal wires. In that case, light particles travel partially across and partially under the wires’ surface; then we call them surface plasmons. They do experience resistance, but scientists can compensate that if they understand what is happening exactly. PhD student Vasco Tenner and group leader Martin van Exter study this phenomenon and publish a new method in ACS Photonics which they use to measure more properties of plasmons, like their phase.

Holes
The Leiden physicists sent a flash of light through a metal surface filled with holes. The interaction with free electrons in the metal hold the light particles captured at the surface, and at the same time the holes keep the light enclosed in a small area. Light particles that want to escape are bounced back by the holes. Now and then a particle accidentally bounces off a hole, away from the metal, so that Tenner was able to measure it. ‘We see that we can learn a lot,’ he says. ‘We already have improved the theory a little.’ The breakthrough gives scientists a new way to study surface plasmons, and eventually use them for faster internet, but also to detect single molecules for diagnosing cancer or to put more details on banknotes.

Leiden physicists send light particles across a metal surface filled with holes. The interaction with free electrons in the metal hold the light particles captured at the surface, and at the same time the holes keep the light enclosed in a small area.

Marileen Dogterom has been elected as member of the Royal Netherlands Academy of Arts and Sciences (KNAW). She is one of the pioneers of the research field of biomolecular physics in the Netherlands. Her research focusses on the cytoskeleton—micro tubes More info

that give living cells their shape and mechanical function, and enable them to divide. In her lab, Dogterom recreates parts of a cell in a controlled environment to get a quantitative understanding of the mechanics behind the cytoskeleton. Her work lays an important basis for the development of artificial cells.

To emphasize her connection with both Leiden University and TU Delft, where she has her lab, Dogterom will hold one of the “Medical Delta” professorships created jointly by the universities of Leiden, Delft and Rotterdam. Medical Delta is a network of life sciences, health and technology organizations that help universities, regional governments, scientists and businesses to overcome cross-disciplinary barriers.

To mark this occasion, Marileen Dogterom will give an inaugural address in the auditorium in Delft, titled ‘Can we build a synthetic cell?’ on June 21 at 3 pm.

The Leiden Institute of Physics hosted the annual conference of the Dutch Vacuum Society (NEVAC) on Friday May 27th. Experts in the field of vacuum experiments talked about their research. The 2016 NEVAC Prize was awarded to PhD More info

student Martijn Vos from the TU Eindhoven.

NEVAC

NEVAC is an organization that promotes the exchange of knowledge in the area of vacuum technology. It sets up scientific meetings, excursions and courses, and awards an annual prize to stimulate young researchers to write about their work using vacuum techniques in a clear and coherent way. Around 60 companies and 300 scientists and engineers are part of the society.

Prize

Martijn Vos won the NEVAC Prize for his publication about a new atom layer deposition process for more efficient solar cells. With his deposition technique, scientists can produce extremely thin layers of special materials—just a few nanometers thick. Vos describes how to make thin layers of molybdenumoxide and how this could be used to increase the efficiency of solar cells.

Johannes Jobst

Johannes Jobst gave a talk on behalf of Leiden University on his measurements of materials that he composes from different layers, like a layered cake, leading to special compositions. ‘There are two options,’ he explains. ‘Either there is interaction between the layers. Then you have created a new material. Or there is no interaction. In that case the layers are isolated and keep their original properties. Sometimes this is what you want, if you need to protect a layer from the environment.’ Together with his colleagues from Sense Jan van der Molen’s research group, Jobst is able to measure this interaction, and count the number of layers that compose their layered cake.

Experimental physicist Jan Aarts has been awarded a FOM Projectruimte grant of 545.000 euro. He will use the money to research the interplay between superconducting currents and ferromagnetism. In the future this could lead to computer memory with negligible power More info

consumption.

One of many ways for computers to store their memory is with the use of ferromagnets. They read and write data by flipping small ferromagnets using electric currents. Inevitably, this goes hand in hand with the loss of energy. Except of course if these ferromagnets are superconducting and let currents flow without any resistance. Physicist Jan Aarts and his team already showed that this is possible by growing layers of the ferromagnet chromium dioxide and inducing so-called supercurrents. However, to use this in real-life computers, thin but wide layers are not useful.

In a recent development, the team learned how to grow wires instead of layers. With the FOM Projectruimte grant, Aarts is now able to hire a PhD student and a postdoc and purchase new equipment, so that he can research supercurrents through chromium dioxide devices based on wires. By studying the interplay between superconductivity and ferromagnetism, he aims to lay the groundwork for future applications, like solid state drives that read and write data without loss of power. Aarts’ research group is at the moment the only group in this research field with the skills to fabricate the necessary wires, giving them a head start on their mission.

Electron microscope image of a chromium dioxide devices based on wires. The green wire is the chromium dioxide ferromagnet. The orange wires are superconductors and are necessary to produce a superconducting current through the green wire.

Vasyl Palchykov from the Lorentz Institute will give an LCN2 seminar on May 27th at 16:00 in room HL214, titled 'Ground truth? Clustering scientific publications'.

Abstract
Community detection techniques are widely used to infer hidden structures within interconnected systems. More info

Despite demonstrating high accuracy on benchmarks, they reproduce the external classification for many real-world systems with a significant level of discrepancy. A widely accepted reason behind such outcome is the unavoidable loss of non-topological information (such as node attributes) encountered when the original complex system is represented as a network.

In this talk we will show that the observed discrepancies may also be caused by a different reason: the external classification itself. For this end we use scientific publication data, which i) exhibit a well defined modular structure and ii) hold an expert-made classification of research articles. Having represented the articles and the extracted scientific concepts both as a bipartite network and as its unipartite projection, we applied modularity optimization to uncover the inner thematic structure. The resulting clusters are shown to partly reflect the author-made classification, although some significant discrepancies are observed. A detailed analysis of these discrepancies shows that they carry essential information about the system, mainly related to the use of similar techniques and methods across different (sub)disciplines, that is otherwise omitted when only the external classification is considered.

Physicist Luca Giomi has been awarded an NWO Vidi grant of 800,000 euro to theoretically investigate artificial cell-like structures. This year, a total of 572 researchers applied for a Vidi. Only 87 of them have been granted funds, including twelve More info

from Leiden University.

The goal of the Giomi’s research is to theoretically investigate an artificial cell-like structure to understand how mechanical functionality emerges in living matter. Living cells are capable of astonishing mechanical functionalities. They can deform spontaneously or, in response to environmental stimuli, move in a fluid or on a substrate and generate enough force to split themselves in two, while remaining alive. These unique properties are possible because the building blocks that form the mechanical apparatus of the cell are active: they are able to transform stored or ambient energy into mechanical work.

But how does functionality emerge from mechanical activity? In order to address this question, Giomi will focus on an artificial cell-like structure consisting of a lipid vesicle enclosing an active liquid crystal that performs the functions of the cytoskeleton. Using this active prototype cell as a model system, he will shed light on the complex mechanical properties that characterize the “fabric of life”.

As part of the master course 'Academic and Professional Skills', four physics master students have created a video about Sense Jan van der Molen's research group. Jamie Culkin, Anne Meeussen, Sander van Lidth de Jeude and Isabelle Jansen devised the More info

project from scratch and started a collaboration with filmmaker Nestor Romero Clemente.

An international team of scientists, including Leiden physicist Dorothea Samtleben, has used the ANTARES and IceCube neutrino telescopes to study the black hole merger that became famous last February as the source of the first ever detected gravitational waves. Any More info

emitted neutrinos would give a more detailed picture of the event than scientists could deduce from only gravitational radiation. Samtleben and her colleagues searched in the recorded data from both telescopes for hints of neutrino outbursts within 500 seconds of the historic merger, coming from the same part of the sky. In the end, they didn’t find any candidate events.

This study is the first neutrino follow-up of a gravitational wave event, and is a good test case for following up the many expected future gravitational radiation detections. It also gives the first concrete limit on neutrino emission from this specific February merger. A total of 1395 authors publish the corresponding paper in Physical Review D.

Rare-earth materials are prime candidates for storing quantum information, because the undesirable interaction with their environment is extremely weak. Consequently however, this lack of interaction implies a very small response to light, making it hard to read and write data. More info

Leiden physicists have now observed a record-high Purcell effect, which enhances the material’s interaction with light. Publication on April 25 in Nature Photonics.

Ordinary computers perform calculations with bits—ones and zeros. Quantum computers on the other hand use qubits. These information units are a superposition of 0 and 1; they represent simultaneously a zero and a one. It enables quantum computers to process information in a totally different way, making them exponentially faster for certain tasks, like solving mathematical problems or decoding encryptions.

Fragile
Now the difficult part is to actually build a quantum computer in real life. Rather than silicon transistors and memories, you will need physical components that can process an store quantum information, otherwise the key to the whole idea is lost. But the problem with quantum systems is that they are more or less coupled to their environments, making them lose their quantum properties and become ‘classical’. Thermal noise, for example, can destroy the whole system. It makes quantum systems extremely fragile and hard to work with.

Electron orbits
Yet, Leiden physicist Dirk Bouwmeester and his colleagues take on the challenge to devise a quantum system to serve as qubit. They plan to use the orbits of electrons around atomic nuclei as ones and zeros. Hitting many atoms with light will move one of the electrons up, giving scientists a way of writing data. This data can be read out with a second light pulse, forcing the electron to move down again, thereby emitting a light particle containing the information. If the atom also interacts with its surroundings, this storage principle does not work perfectly because part of the information is lost to the environment. First author Dapeng Ding uses so-called rare-earth ions to avoid this quantum information leak. These particles can serve as stable storage for as long as ten seconds—an eternity in the otherwise very fragile quantum world. In comparison: other commonly used systems for quantum computer research decay within microseconds—over a million times more rapidly.

Purcell effect
Alongside their incredible stability, rare-earth ions come with a problem; they interact only very weakly with light, making it difficult to write and read data. To resolve this problem, the physicists trapped light together with rare-earth ion ytterbium (Yb3+) in a so-called ring resonator. Much to their satisfaction, they saw that the ring resonator induced the Purcell effect, which enhances the interaction with light. This offsets the major pitfall of the use of rare-earth ions, and paves the way for Bouwmeester’s proposal to improve storage of quantum information.

Vincent Traag from the Centre for Science and Technology Studies will give an LCN2 seminar on April 29th at 16:00 in room HL214, titled 'Methods & algorithms for detecting communities in large networks'.

modular structure: groups of densely connected nodes with few connections between the groups. Nodes in such groups often have something in common, and enrich our understanding of complex networks. Finding such so-called communities in large networks is far from trivial. One of the best-known methods for community detection is modularity, which specifies a quality function of a partition. However, modularity suffers from a well-known flaw, known as the resolution limit: it tends to oversimplify, and lump together several (sub)communities in one large community. We here show that only few quality functions can address this issue. One of the best algorithms for optimising modularity is the Louvain algorithm. We here show that it can lead to arbitrarily badly connected communities---in addition to the resolution limit of modularity. In particular, it can lead to disconnected communities. We here introduce a new algorithm, and show it not only addresses this caveat, but also that it asymptotically ensures that no subset of any community can be moved to another community. Finally, we introduce a fast local move subroutine, speeding up the algorithm 5-10 times.

On May 1st, we are delighted to welcome Prof. Charles Kane as this year’s Lorentz Professor. Kane is a world-renowned American physicist who has made major contributions to theoretical condensed matter physics. He is most famously known for discovering topological More info

insulators in the mid-2000’s. For his work on this subject he received the Dirac Medal of the International Centre for Theoretical Physics in 2012, along with two colleagues.

During his stay in Leiden, Kane will give a series of three lectures titled 'Symmetry, Topology and Phases of Matter' in the De Sitter room (Oort building).

Abstract
The lectures will provide a pedagogical introduction to topological band theory and discuss its application to symmetry protected electronic states including topological insulators, semimetals and superconductors. Would it be possible to apply related ideas to mechanical systems?

Milan Allan’s research group has created a timelapse of their efforts to build their scanning tunneling microscope (STM), called Tamagotchi. An STM allows researchers to see the electronic structure of materials with atomic precision. Allan’s group wants to understand the More info

strangeness of exotic quantum materials, and use their STM for that purpose.

The European Physical Society has awarded the prestigieus Edison Volta Prize to Leiden physicist Michel Orrit. The prize is handed out every two years to an individual or a group of at most three scientists as an acknowledgment of an More info

outstanding achievement in physics research.

Orrit receives the award for his seminal contributions to optical science and to the field of single-molecule spectroscopy and imaging. He became the first person ever to detect single molecules by fluorescence and he made the first optical detection of magnetic resonance in a single molecule. for pioneering investigations into the photoblinking and photobleaching behaviors of individual molecules at the heart of many current optical super-resolution experiments. Moreover, the jury values his pioneering investigations into the photoblinking and photobleaching behaviors of individual molecules at the heart of many current optical super-resolution experiments.

Orrit joins an exclusive group of Edison Volta Prize winners, including the discoverers of the Higgs particle and the scientists who made the most recent, spectacular map of the cosmic microwave background. Recently he also won the Physica Prize and the Grand Prix SFO.

Leiden physicist Alexey Boyarsky has been awarded an ERC Advanced Grant to research an extension of the standard model of particle physics. He shares the grant with research groups from Lausanne and Copenhagen. 580.000 euro goes directly to Leiden.

his colleagues Prof. M. Shaposhnikov from the Technical University of Lausanne and Prof. O.Ruchayskiy from the Niels Bohr Institute in Copenhagen aim to create an extension of the so-called standard model of particle physics, which in its current form is inconsistent with some observations in the Universe, like the presence of dark matter or the large dominance of matter over antimatter.

One of the peculiar features of the standard model is that it only contains left-handed neutrinos. This is an important aspect, because these lightweight particles played a major role in the evolution of the Universe during the period right after the Big Bang. The international collaboration now wants to investigate an extension with right-handed neutrinos.

The team will make predictions about the signatures that scientists might see in their data obtained from both particle accelerators and telescopes directed at far away galaxies. If those signatures would indeed be visible, it means the confirmation of the team’s proposal. In that case, the problems that physicists have with the standard model are solved.

Aside from making theoretical models, Boyarsky will analyze observational telescope data himself as well. He needs to be patient, though, for any confirmation from particle accelerators, as his Publ. 12-04-2016 14:52

Physicist Jelmer Renema receives an NWO Rubicon grant to conduct a 2-year postdoctoral research on a proof of concept for quantum computers at the University of Oxford.

Renema earned his PhD in Leiden and uses his Rubicon to move to Oxford. More info

He tells us enthusiastically about his research.

‘My research is about building a non-universal quantum computer. This means a quantum computer that can only perform a specific calculation. The motivation behind this is that building a fully functioning quantum computer appears to be extremely difficult. It is a project the size of the Manhattan project, or building the first nuclear fusion reactor. So this crosses an entire generation.

‘For that reason, it’s interesting to see in the meantime if we could build a machine that can execute some calculation—which is difficult in “classical” terms—more efficiently based on quantum technology. Up until now, our primitive quantum calculations have always been easily imitated by a classical computer. For example, if you want to factorize the number 15 in prime factors, using a quantum computer this will take years of work plus a lab full of people, while you could easily take a pen and a piece of paper and quickly find that 3 multiplied by 5 gives 15.

‘Our goal is to make a quantum calculation which is not that easily reproducible on a regular computer. That calculation doesn’t even have to be promising in terms of applications. It’s all about showing that a quantum calculation can in fact be faster than its classical counterpart.

‘There is a specific proposal for such a non-universal quantum computer; the Boson Sampler. A Boson Sampler is a network of certain glass fibers, that are placed in a glass plate. When two of those fibers meet, a photon (light particle, edit.) could jump over from one fiber to the other. Now if there is a photon in each of the fibers, they will influence each other’s path in a way that can only be described by quantum mechanics.

‘This leads to a level of complexity that rises above classical capabilities. But in the experiment this happens automatically: the machine calculates itself efficiently. My ultimate goal is to extend the existing Boson Sampler to a version that is large enough so that a classical computer won’t be able to keep up with its speed. Then, we have demonstrated that quantum calculators are indeed fundamentally different from classical computers.’

Rubicon is an NWO finance instrument for scientists who have recently obtained their PhD, to gain experience at foreign top institutes, as stepping stone towards a scientific career.

The institute of physics celebrates the completion of two long-term projects. The renewed facility for the production of liquid helium is all done, and the low-vibration measurement island in the new science building performs at world-class level.

designing, testing and adjusting, the brand-new low-vibration physics lab at the new science campus is working like a charm and ready to make its mark in the world of science. The construction is supported by an enormous, 1-meter thick concrete floor and gigantic damped springs, which almost entirely absorb every hint of vibration. This is of paramount importance for researchers, who execute precision measurements at the atomic scale in the ice-cold silence of temperatures close to absolute zero. Without these extreme measures, every speeding truck on the A44 highway ruins the whole experiment.

World-class

Over the last couple of years, research technician Marcel Hesselberth has put much time and effort in getting the measurement island as quiet as possible. In the end, he managed to make the low-vibration lab one of the most silent in the world. This is an ever bigger achievement if you consider Leiden’s soil; peaty clay is notoriously difficult to build on. In his presentation, Hesselberth explains the mechanisms behind the ingenious construction, and how he brought down the vibration level to over a hundred times that of the current physics building. ‘Our measurements were so sensitive that we could even tell the force of the wind outside,’ he says while the big screen shows two equal graphs of the registered vibrations and the official wind force numbers of that moment. The new science campus is now on the exclusive list of the best low-vibration labs in the world.

Liquid helium

The renewed liquid helium facility is still located in the Huygens building and much less affected by vibrations. Yet, this installation was in need for improvement as well. The old one had reached a certain age and started showing several defects. On top of that, the construction was not suited to move to the new campus. The new facility has lower production and personnel costs, a smaller impact on the environment and offers the possibility to adjust the production quantity.

Image caption: LION research technician Marcel Hesselberth poses next to one of the helium tanks during the celebrations in the new facility for liquid helium.

Manufacturers produce high-end technology mostly top-down with large machinery, but small particles are able to build structures by themselves from the bottom up. A major challenge is that these particles easily clump together. Leiden physicist Daniela Kraft has developed a More info

method to use this phenomenon to her advantage. Publication in ACS Nano.

Building blocks
Smaller computer chips, narrow sound boxes, miniature cameras; we keep aiming for smaller and more complex technology, to carry with us or to use for surgery. At the same time, it gets increasingly hard to build a complex structure on an even smaller scale. Wouldn’t it be much more convenient to build structures bottom-up, starting from tiny building blocks? That is exactly the idea within the research group of Leiden physicist Daniela Kraft. She is working on a method to build structures from colloids— particles that are larger than nanoparticles but too small to see with the naked eye. And the fun part is that colloids operate completely on their own, as independent building blocks.

Chunks
This field of research is still in its early stages, but Kraft and her PhD student Vera Meester have now made a giant leap forward by developing a method to use a large barrier to their advantage. ‘Colloids have a strong tendency to clump together,’ says Kraft. ‘Normally speaking that is bad news, but we let them go ahead and make sure that they rearrange into a desired structure afterwards.’

Control
They control the building process by adding salt or oil to the colloidal solution at specific times. This enables them to control the attractive Van der Waals forces and the surface tension. Under the influence of these forces, the randomly shaped chunks swell and reconfigure in a specific way. The type and concentration of salt and oil determine which structures the colloids form. By testing different combinations, Kraft now knows how to create a number of basic structures, from a simple dumb-bell shape to a pentagonal dipyramid. ‘Theoreticians have already predicted what kind of useful larger structures we can build with these basic building blocks, but in practice you never know what is actually going to happen.’

Medical robots
Once physicists have obtained sufficient knowledge on how to command colloids to build specific structures, they will bypass the limit that manufacturers approach from their top-down approach by going the other way: bottom-up. In this way they’ll be able to fabricate miniature devices that are out of reach for the conventional industry. Kraft: ‘In the future we might build tiny light switches, or medical robots. Because we work bottom-up, we won’t be limited with respect to complexity, materials or length scales.’

When researchers add oil to colloidal clumps, the particles recycle themselves into uniform complex base structures. All clumps containing 2 particles form dumbbell shapes, 3 particles form triangle shapes and 4 particles make up tetrahedron, like we see in this electron microscopy image.

Leiden physicists have managed to detect a single molecule called dibenzoterrylene in a new crystal, and found that it is a candidate component for a quantum network. Future quantum computers will need such a network to work together while maintaining More info

Quantum computers hold a big promise for the future. With exponentially faster calculations they should be able to solve scientific problems and crack codes that are currently untouchable for modern computers. And to fully exploit the potential of quantum computers they would have to be connected through a quantum network. In regular networks the so-called qubits have to transform into ordinary bits in order for a classical wire to carry them. That way the speed advantage of the quantum computer would be completely lost.

Individual molecule
Leiden physicist Michel Orrit studies single molecules as candidate building blocks for the necessary quantum network, using a technique that he developed in the early ‘90s and which made him the first person to see an individual molecule with the use of fluorescence. Orrit’s invention laid the groundwork for a follow-up technique that went on to earn the 2015 Nobel Prize in Chemistry. And to this date, his original technique still proves to be valuable in modern research. Apart from bringing him the 2016 Physica Prize, it enabled Orrit to detect an individual dibenzoterrylene molecule, together with PhD student Nico Verhart, and check its properties for use in quantum networks.

Radio channel
The researchers point a laser beam on their sample, which is extremely diluted—100 nanomole per liter—leaving only a few molecules in focus for the laser. Needless to say, temperatures are just above absolute zero, so the particles stay neatly in place. Slowly changing the laser’s color, they try out many different frequencies to which a molecule might tune in. It is similar to a radio show trying out several broadcast signals to match the frequency that your radio is tuned in to. Every molecule is programmed to only interact with light of a very specific frequency, like an old radio that is stuck to one channel. When it gets hit by light of exactly the right color, it absorbs and emits those light particles; it fluoresces.

Stable
Once the physicists have stumbled upon a matching color, they study the stability of the fluorescence signal. The more stable the signal, the better equipped the molecule is to serve as building block of a quantum network. The frequency of an instable molecule will drift away over time, for example, or the molecule could enter a dark state, in which it doesn’t fluoresce at all for a while. It turns out that dibenzoterrylene molecules present neither of these instabilities and are good candidate building blocks for a quantum network.

Thomas Schmidt, biophysics professor at LION,
and Erik Danen from the Leiden Academic Centre for Drug Research
organize an international meeting on mechanobiology in
Amsterdam from March 22 to 24. Scientists in this field study how
mechanical cues contribute to cell development, cell More info

function and disease.

The Mechanobiology 2016 meeting gathers scientists from different disciplines, from physicists to
clinical researchers,
in order to discuss the latest insights in how cells sense and respond to
mechanical signals. These insights should lead to a better understanding of diseases like fibrosis, heart failure and
cancer.

Room 111 in the Huygens building is fully renewed and upgraded to a brand new furnished room for classes and self-study, called Studio Classroom. It is equipped in accordance with a concept developed by TU Delft. Studio Classroom allows for More info

both ‘classical’ centralized teaching and decentralized group or individual learning. Teachers have a large touchscreen at their disposal and students can work together on mushroom shaped tables with at least two computers, but also space for offline activities to encourage discussion.

The afternoon of Tuesday March 22nd is reserved for a walk-in session between 14:00 and 17:00. Anyone can drop by to feel the room, play around with the touch screen and ask questions. Interested teachers can subscribe for a 2-3 hour training in April to fully exploit the new possibilities. They are asked to send an email to This email address is being protected from spambots. You need JavaScript enabled to view it. before March 25th.

Booking & Availability
- Room reservation for educational activities and group meetings is coordinated through This email address is being protected from spambots. You need JavaScript enabled to view it.
- Availability is visible on digital screens on the ground floor at the reception of the Huygens building.

AV & IT support
- Studio classroom has eight workstations, each equipped with seven chairs.
- Each workstation has one PC screen that is connected to the main touch screen.
- Glass windows function as a whiteboard; special markers are available next to the boards.
- For IT support, contact Erik Deul.

Wim van Saarloos will become vice president of the Royal Netherlands Academy of Arts and Sciences (KNAW), and return to the Leiden Institute of Physics as professor of theoretical physics. Currently, Van Saarloos is Transition Director at the Netherlands Organisation More info

for Scientific Research (NWO).

After studying Technical Physics at the TU Delft, Van Saarloos started a PhD track in Leiden with Peter Mazur, which he completed in 1982. He went on to work in the United States for AT&T Bell Laboratories, before returning to Leiden in 1991, where he stayed for 18 years as professor of theoretical physics. From 1997 onwards, he was also director of the university’s Lorentz Centre. Van Saarloos left Leiden in 2009 to become director of the Foundation for Fundamental Research on Matter (FOM). As of last June, he has been conducting his current job at NWO.

Van Saarloos has conducted ground-breaking research in theoretical physics, among others on the instability of wave fronts and granular materials. He has received national and international recognition for his scientific work, including his appointment as fellow of the American Physical Society (2007) and the Dutch Physica Prize (2008). He became a member of KNAW in 2004. At the moment, Van Saarloos is member of Leiden University’s supervisory board, but he will step down when returning to LION.

Medical doctors see lumps of the protein Aβ-peptide in Alzheimer patients’ brains. They are not sure however if this causes the disease, or if it’s a consequence of the damage these proteins do on brain membranes. Resolving this matter is More info

Martina Huber and her research team managed to detect how Aβ-peptides behave around a mimicked brain membrane. She put a soapy substance in an Aβ-peptide solution, causing the peptides to assume the presence of a closeby membrane. With a so-called EPR tracing technique Huber kept track of the proteins’ location, giving her information on whether they immediately started clumping together, or if they first cut off a piece from the imitation membrane.

It depends on the ratio peptide/soap what will happen. With a high concentration of peptide and little soap, there is more binding between both substances, which we associate with membrane damage. On the contrary, a low ratio leads to more lumps. Huber performed her first test on unrealistic concentrations, meaning she can’t give an answer for the human body. But she does offer scientists for the first time a method to answer the pressing question for realistic ratios.

‘Now we can start studying which ratio leads to which scenario, and which of those ratios actually occur in our body,’ says Huber. ‘We already see that the protein lumps are less hydrophobic than we expected. We used to suspect that these Aβ lumps refuse to mix with water. Now they appear to be able to access the membrane at more spots than we thought.’

The importance of Aβ-peptide research is demonstrated by the fact that it has no use in normal, healthy processes. Huber: ‘Our body only produces it by mistake. It has no legitimate excuse to be present in our brains. So it is clear that this protein has something to do with Alzheimer’s. And now we have the means to find out what it does exactly.’

The European Strategy Forum on Research Infrastructures (ESFRI) has selected KM3NeT for its 2016 Roadmap.

The neutrino telescope KM3NeT is an international research facility with scheduled locations on the bottom of the Mediterranean Sea, near Toulon (France) and Sicily (Italy). At More info

the end of last year, scientists installed its first detector. KM3NeT has three main goals: discovering astrophysical sources of cosmic neutrinos, determining the relative masses of neutrinos and facilitating ocean and environmental research. Neutrinos are neutrally charged elementary particles that interact extremely weakly with matter and are therefore hard to detect. An advantage is that physicists can directly pinpoint where they originate from, because nothing will have deflected them on their journey. The place on the ESFRI Roadmap means acknowledgement of both KM3NeT’s scientific importance and its technical feasibility.

The Netherlands plays an important role in the project, with among others Dorothea Samtleben from the Leiden Institute of Physics and Nikhef. The proposal to put KM3NeT on the roadmap was presented by the Dutch ministry of Education, Culture and Science (OCW), with political support from Greece, France and Italy.

Note:
ESFRI is a strategic body of European governments to promote scientific integration within Europe. Setting up large international research facilities requires European collaboration and a coherent strategy, which has been established for the upcoming years in the ESFRI Roadmap. To be eligible for a place on the roadmap, a facility should have political and financial support from at least three countries. 21 projects are part of the ESFRI Roadmap.

March 14th is a special day for science enthusiasts worldwide, as we not only celebrate π day but also Albert Einstein’s birthday. To commemorate this anniversary and encourage as much π jokes as possible, the Leiden institute of physics organizes More info

an afternoon for high school teachers and students. The day is filled with lectures on Einstein’s life in Leiden, his role in quantum mechanics, his theory of general relativity and the recent detection of gravitational waves. The participants will also get a guided tour through many different labs, like the Kamerlingh Onnes lab, the Cell Observatory and the Electron Paramagnetic Resonance lab. A staggering 42 teachers and 121 students have already subscribed, exceeding the capacity of the De Sitter room. For this reason an additional live stream is set up in room 111.

Science Magazine publishes three back-to-back papers on an important discovery in solid state physics. Leiden physicist Jan Zaanen writes a Perspective article on the matter in the same issue of March 4th.

discovered that electrons behave like a fluid when flowing through certain materials. Until now, scientists had only observed this effect in extremely exotic circumstances, like the conditions at the time of the Big Bang. It is the first time that physicists see electrons flowing as if they were a stream of water in ‘everyday’ materials—in this case graphene and PdCoO3.

If you turn on a gardening hose, you not just initiate a cross fire of individual water particles, but an actual stream of liquid water. Water is different from the sum of its parts. It behaves as a liquid, with phenomena like waves, turbulence and viscosity. When switching on the lights, however, you only set in motion a series of single electrons, like bullets from a gun. Electricity normally doesn’t behave like a fluid.

But the beauty of physics is that things oftentimes aren’t as normal as they seem at first sight. In three independent studies and using different materials, researchers found that electrons can actually mimic water. And astonishingly they did so at practically the same time. When electrons form a strongly interaction system, they act collectively as a fluid, provided that temperatures are not close to absolute zero. As a leading expert in the field, Jan Zaanen publishes a Perspective article on the discovery in Science.

Zaanen sees great potential in the richness of electron flows, compared to traditional electric currents. The shocks and turbulence that are inherent to so-called hydrodynamics might give future technology a whole new dimension. Also for research purposes the finding is of significant value. For example, it enables scientists to test their theories about the physics behind strange metals in the laboratory. These materials are made up of a vibrant soup of extremely interacting particles that scientists have very little knowledge about, and which probably will reveal fundamental laws of nature.

A new discovery proves that it matters which approach researchers take in analyzing large physical, social or biological systems that have a networked structure. Ever since the early 1900s, scientists have assumed each approach is equivalent. Now many results in More info

Two approaches
Using complex models, scientists analyze large systems like fluids, social structures, wildlife interactions or the global economy. These have so many units that researchers can only describe them in terms of probability. They keep the exact microscopic details unspecified and assume the system is in any of the equally likely configurations compatible with a handful of known and measurable macroscopic constraints, like the total energy. There are two main trains of thought for doing this. Either the constraints are considered as strictly fixed, or some margin is allowed on their value. We call these approaches microcanonical and canonical, respectively.

Proof
For over 100 years, scientists have assumed that for analyzing large-scale systems with so-called short-range interactions, it doesn’t matter which approach you take, as both lead to the same result. Now Leiden physicist Diego Garlaschelli, mathematician Frank den Hollander and their groups have come up with a different scenario, proving that the microcanonical and canonical approaches can be fundamentally different. The examples they considered come from network theory, where the units in a system are nodes in a network, with some constraint either on the total number of links between them or on the number of links per individual node. In the former case the approaches are equivalent as expected, but now it surprisingly turns out that in the latter case they are not, meaning that it actually matters which approach you take to analyze the system.

Epidemics
Besides its theoretical novelty, the result has important practical implications for the choice of approaches in studying global epidemics or financial crises. ‘Let’s say we look at the social interactions in the spread of epidemics,’ says Garlaschelli. ‘We don’t know for every person the exact amount of connections they have with other people. So for some, we need to make an educated guess. In the microcanonical approach, we would pick a fixed amount of connections for each person, and strictly hang on to it. Now if we make one wrong guess for a person, this error would propagate through our model and we’d make a correlated error with another person. But in the canonical approach, we automatically allow for such uncertainties and one wrong guess doesn’t set off a chain reaction of wrong implications. Our new proof implies that you should choose the canonical one in this case.’

Fundamental importance
The example may sound straightforward, but the research team had to carry out accurate calculations combining statistical physics and discrete mathematics to prove their claim. ‘The discovery is very important at a theoretical level for our understanding of the fundamentals of statistical physics’, Garlaschelli explains. ‘Many results of statistical physics, which underlie much of our understanding of large real-world systems, may no longer hold.’

This Saturday March 5th, Leiden University organizes its bi-annual open day in the city centre. Participants can gather information about all the bachelor studies that Leiden has to offer. There is plenty of opportunity to ask all questions you might More info

The small quantum world and our world of perception obey different laws of nature. Leiden physicists search for the border between both worlds. They set an upper limit in an article published soon in Physical Review Letters.

of nature in the domain of quanta do not apply to our everyday lives. We are used to assign an exact location and time to objects. But fundamental particles can only be described by probability distributions. As if the police writes out speeding tickets for driving 30 to 250 km/h somewhere between Paris and Berlin, with a probability peak for 140 km/h in Frankfurt.

Boundary
Because the laws are completely different in both worlds, a clear boundary might exist between them. Then you would want to determine per object whether it obeys quantum or macroscopic laws, depending on size and mass. Still, it has been a mystery where this frontier lies exactly. Leiden physicist Tjerk Oosterkamp and his research group establish an upper limit, closing in on the answer.

‘We keep excluding values, so that we slowly close in on the boundary’s location,’ says Oosterkamp. ‘If we only have a small area left, we can better design our experiments to see what is happening at the edge of the quantum world.’

Parameters
According to a certain quantum mechanical model, you can describe a particle’s position with a probability distribution that sometimes spontaneously ‘collapses’. In that case its position is indeed determined precisely, within a certain margin. This margin and how often the spontaneous collapse occurs, form the two parameters that physicists are after. If they find those, they have a complete formula to define a strict border between quantum and macro.

Now Oosterkamp has determined an upper limit on these parameters of 31 collapses per year per atomic mass unit with a margin on the location of 10 nanometers, to 1 collapse per 100 years with a margin of 1 micron. For their next measurement they expect fewer collapses, so they can define an even stricter upper limit.

The red line shows the upper limit for parameters rc (margin on location) and λ (frequency of wave function collapse per atomic mass unit). The black dotted line gives the expectation for Oosterkamp’s next experiment. The purple ball estimates the values based on the fact that electrons definitely behave quantum mechanically around large molecules. In the end, the goal is to found the values for both parameters. This completes the formula for the boundary between the quantum and macroscopic world. For a given size and mass, this formula tells which laws apply to the object.

The financial system needs complexity theory to predict economic crises like the 2008 meltdown. An international team of scientists, including Leiden physicist Diego Garlaschelli, state this in a paper published in Science on February 19th.

proven to fail in explaining, let alone predicting the 2008 financial crisis. Complex models used to study ecological systems and the spread of diseases are indispensable for a stable banking system, according to scientists from the University of Amsterdam, Wageningen UR, Utrecht University, Leiden University, University of Groningen, Oxford University, ETH Zürich, Santa Fe Institute, Bank of England and the Global Climate Forum.

Prediction
Most crises are not caused by sudden events, but by slow dormant processes. With complexity theory, scientists could identify these and use them to predict and prevent economic collapses. Earlier research from Garlaschelli, in collaboration with De Nederlandsche Bank, shows that the 2008 crisis came as a surprise to traditional models, but a more realistic model accounting for different bank sizes could have predicted it three years in advance.

Individual nodes
Statistical physicists, ecologists and epidemiologists have tools to analyze the stability, robustness and resilience of extremely complex natural systems that have large numbers of actors. They identify interactions, measure their strength and study how chains of events propagate. The banking system is just as convoluted, calling for banks to be considered as individually different nodes in a large network. Some banks have for example a more central role than others, meaning the system’s stability depends more on them. Another hazard is information asymmetry, where banks are unaware of other banks’ issues. ‘Even central banks don’t have complete information about the system’, says Garlaschelli. ‘My research also focusses on the optimal reconstruction of financial networks from partial information.’

Tipping point
The key for a good warning system is to notice a so-called tipping point well in advance. Social experiments have shown that economic systems tend to have cascade effects leading to extremes like real-estate and stock market bubbles. Eventually this leads to a tipping point, after which a crisis is inevitable. On the contrary, in order for a system to be healthy, it should have mechanisms restoring equilibrium. Garlaschelli: ‘Recent experiments in Cars Hommes’ group in Amsterdam have shown that enabling such stabilizing mechanisms is indeed possible in real economic systems.’

The authors conclude that funding required for a warning system based on complexity theory stands in no comparison to the cost of an economic crash.

Top graph: Traditional model. Deviating number of intertwined banks based on the total number of links in the financial system. Bottom graph: More realistic heterogeneous model. Deviating number of intertwined banks based on the number of links per bank individually. The bottom graph shows a prediction of the 2008 crisis, whereas the top graph only declines at the start of the crisis. Note: Intertwined banks are pairs of banks that are at the same time debtor and creditor of each other. [Modified from Squartini, van Lelyveld, Garlaschelli, Scientific Reports 3:3357 (2013).]

Assaf Almog from the Leiden Institute of Physics will give an LCN2 seminar on February 26th at 16:00 in room HL106, titled 'From the brain to the economy: finding communities in networks and correlation matrices'.

of complex systems, from financial markets to the brain, is an intermediate between the microscopic dynamics of individual units (stocks or neurons, in the mentioned cases), and the macroscopic dynamics of the system as a whole. Indeed, many systems tend to organized in a modular way, with functionally related units being correlated with each other, while at the same time being relatively less (or even negatively) correlated with functionally dissimilar ones. The empirical identification of such emergent organization is challenging due to unavoidable information loss, when inferring the structure from the original time series activity data.

In this talk, I will present a modularity based community detection approach for correlation matrices. The method uses maximum-entropy null model designed specifically for correlation matrices (and not networks) that is able to filter out both unit-specific noise and system-wide dependencies. This results in identification of meso-scale functional modules that are internally correlated and mutually anti-correlated. I will present applications to brain networks, financial markets, and international trade.

Lastly, using the maximum-entropy framework, I will discuss a new null model for community detection. This "enhanced" null model is able provide link expectations based on both the strengths and the topology of the network. The application of this model to the International Trade Network reveals differences with respect to the standard approach.

Tjerk Oosterkamp is awarded an NWO Large Grant of 2.5 million euro to build a machine that offers an environment with ultra-high vacuum and very low temperatures, only one thousandth of a degree above absolute zero. This will provide many More info

research groups with the opportunity to conduct groundbreaking research under unprecedented conditions. The facility will be unique in the world and its future place in the low-vibration lab at the new Leiden science campus gives it even larger capabilities.

On the level of the quantum world at room temperature, particles swarm and vibrate wildly. Physicists learn about this miniature world by very precisely studying a small sample plate containing ‘entangled’ particles. Surely they are unhappy with the chaotic situation at room temperature that quickly destroys the very fragile entwined state that they want their particles to be in. Just imagine gluing a broken pot back together in a hurricane. And without a proper vacuum, the sample plate will get contaminated immediately by unwanted atoms on the sample surface. So add a dust storm to the hurricane scenario.

Quantum physicists need extremely low temperatures and an ultra-high vacuum to perform their research. Only then, their sample will stay clean and its particles stay entangled. Oosterkamp’s new machine will offer exactly that. In just over a year, physicists can come to Leiden to:

- Investigate unexplored quantum effects at ultra-low temperatures
- Explore new measurement methods to detect phenomena of intertwined electrons in materials that are said to ‘quantum compute all by themselves’
- Explore the infamous quantum measurement problem in the context of systems containing highly entangles particles

The 2.5 million euro vacuum fridge offers its state-of-the-art environment to a variety of instruments: scanning gates, scanning tunneling microscopes, atomic force microscopes and magnetic resonance force microscopes.
A schematic drawing of the system, featuring a dilution refrigerator which offers space for up to four scanning probe microscopes at the coldest plate at the bottom (10 mK with nuclear demag possibilities to Publ. 11-02-2016 17:49

An ambitious plan to add a new angle to the hunt for dark matter has passed an important landmark. A collaboration of particle physicists and theorists has proposed an experiment to try and create ‘sterile’ neutrinos with the so-called Super More info

Proton Synchrotron (SPS). Leiden physicist Alexey Boyarsky was among 15 authors of the initial letter of intent that resulted in the creation of the collaboration. The SPS is part of the particle lab CERN in Geneva. The SPS council has now endorsed the experiment, meaning that the proposal will go into a three year process of testing. If the CERN council finally approves, the 185 million euro Search for Hidden Particles (SHiP) Experiment will kick-off in 2026.

Boyarsky is on a quest to unravel the nature of one of the most mysterious phenomena in the Universe. The cosmos appears to consist for the larger part out of mass we cannot see. We have no idea where it comes from. Physicists call this ‘dark matter’. Sterile neutrinos are candidate building blocks of dark matter. However, it has never been proven that these hypothetical particles even exist at all.

If they do exist, they would have an extremely weak or even no interaction with ordinary matter. The proposed SHiP experiment uses SPS’s high-intensity proton beam to try and produce heavy sterile neutrinos, which should occasionally decay into normal matter. The collaboration expects the protons to create the sought-after particles through a chain of events, after which they would be able to indirectly prove their existence by measuring signature decay signals.

The SHiP collaboration reached an important landmark in their attempt to set up a 185 million euro experiment that uses this Super Proton Synchrotron at CERN to prove the existence of sterile neutrinos.

Michel Orrit is honoured with the Physica prize 2016 for his groundbreaking work on single molecule spectroscopy. In the mid ‘80s, Orrit came to the realization that it should be possible to optically detect a single molecule. A few years More info

later, in 1990, he indeed became the first one to detect the fluorescence signal of one molecule.

Last year, the Nobel Prize in Chemistry was awarded to Betzig, Hell and Moerner for the development of super-resolved fluorescence microscopy. The Nobel Committee’s description of the scientific background clearly showed the groundbreaking significance of Orrit’s experiment as the basis for the super-resolution techniques that were established afterwards. Moerner measured a single molecule slightly before Orrit, using absorption, but Orrit’s measurement using fluorescence produced much less background noise and became the standard in this scientific field.

Orrit’s work gave rise to a whole new research area; single molecule/particle optics. Since he started working at the Leiden Institute of Physics, he has built up a very active research group. Recently they developed a ‘nano microphone’—a microphone consisting of only one molecule.

For the first time, scientists have entangled four photons in their orbital angular momentum. Leiden physicists sent a laser through a crystal, thereby creating four photons with coupled ‘rotation’. So far this has only been done for two photons. The More info

Entanglement holds a great promise, with applications in perfectly secret communication and quantum computing. If two photons are created simultaneously, they are each other’s counterpart, so that their ‘rotation’ is always reversed with respect to the other. If we measure left ‘rotation’ for one photon, then the other will always ‘rotate’ to the right after measurement with a similar filter. This is called entanglement. Before the measurement, each photon’s ‘rotation’ is undetermined.

Milestone
This ‘rotation’ is a property of photons that scientists discovered in 1992 in Leiden; physicists call this orbital angular momentum. And this property has more than two values. It covers an infinitely large alphabet of information. So with this you can transfer much more information per photon than with a property like polarization, which contains only two possible values. In 2001, scientists managed to entangle two photons in orbital angular momentum for the first time. Now, Leiden physicist Wolfgang Löffler and his colleagues are the first ones to entangle four photons in this way. They announce it in an Editor’s suggestion article in Physical Review Letters. The discovery offers many extra possibilities, like sending an uncrackable encrypted message to more than one party.

Experiment
During their successful experiment, the researchers sent short ultraviolet laser pulses of two picoseconds through a crystal. Occasionally this leads to the creation of four entangled photons. This is extremely rare, but by generating 80 million pulses per second they managed to detect on average two so-called photon quadruplets each second. To confirm these were indeed entangled in orbital angular momentum, the team used a spatial phase modulator that converts this ‘rotation’ back to light travelling as a plane wave. They registered this ‘normal’ light with single photon detectors.

Leiden physicists sent short ultraviolet laserpulses of two picoseconds through a crystal. This leads to the creation of four photons that are entangled in their orbital angular momentum—here depicted as red blue spirals. The rainbow colored circles illustrate the phase (color) and intensity (brightness) of the photon’s cross section.

Gathier kicked off the evening by explaining the way in which ESA missions are designed and approved. The obligatory contributions of the member states add up to 500 million euro, but additional spending pushes ESA’s budget to 4.4 billion. This money is divided over space missions of various scales. Large missions cost typically 1 billion euro and are led by ESA headquarters about once every 6 years. Medium-sized programs cost 550 million and are executed at twice that frequency. This still leaves a large budget for several smaller missions of different magnitude. ESA’s long-term program is built around four main questions; ‘How does life form?’, ‘How does our Solar System work?’, ‘What are the fundamental laws of physics in the Universe?’ and ‘How was the Universe created?’.

After the dinner break, Henk Buisman took the stage to introduce his new website on quantum classroom experiments. Teachers can select an interesting experiment and take their students to LION to learn about the quantum world through enquiry-based learning.

Martijn van Calmthout spoke about his motivation to write his new book ‘Echt Quantum’, in which he explains quantum mechanics to none other than Albert Einstein and Niels Bohr. Since their deaths mid twentieth century they missed out on several important discoveries, so they have some catching up to do. Van Calmthout meets these two greats at a historic location; hotel Metropole in Brussels, famous for organizing the Solvay conferences.

When the evening turned into night, Tom Westerhof elaborated on his project to build the new building for Leiden’s science faculty. On February 23rd of this year the first phase will officially be completed, to be followed in 2020-2021 by the completion of the second phase, and eventually a third one. Among others, the building will contain an almost vibration-free lab, enabling scientists to conduct experiments with great precision. Zuid-Holland’s soft clay soil makes this extra difficult. Still, the lab will experience the second least amount of vibrations in The Netherlands.

An increasing amount of questions about the quantum world gives a new push to research close to absolute zero. Quantum physicists are moving back to the domain of ultra-low temperatures, where atoms nearly come to a standstill and are therefore More info

easier to study. In line with Leiden tradition, scientists from the Leiden Institute of Physics (LION) have built a fridge that approaches absolute zero up to one milliKelvin. Tjerk Oosterkamp now uses his device—recently on tv show Het Klokhuis—to perform his first measurements on diamond.

Back in 1908, it was Heike Kamerlingh Onnes who became the first person ever to liquefy helium, in his lab in Leiden. For this he needed a temperature of minus 269 degrees Celsius—only four degrees above absolute zero. This earned him the Physics Nobel Prize in 1913. Following his footsteps, LION spin-off Leiden Cryogenics keeps the tradition alive by making fridges that reach temperatures of eight milliKelvin. This provides physicists nowadays with the opportunity to simply buy a device and do what was such a great endeavor for Kamerlingh Onnes. In that sense, life became a lot easier for them. But researchers wouldn’t be researchers if they didn’t want to go further into the extreme. And so Oosterkamp started working on his fridge—together with Leiden Cryogenics—to get it as cold as one milliKelvin. In that intense cold it is possible to conduct groundbreaking science. ‘Ultra-cold research is back and booming,’ he shouts with his device buzzing loudly behind him. ‘For a long time it has been considered less interesting to go towards absolute zero. But now we have many questions that can only be answered in extreme cold.’

Research
Close to absolute zero, particles practically stop moving, so scientists can perform accurate measurements. For example on new combinations of materials they want to use in future quantum computers. Or they dive into questions about the laws of nature on the interface of quantum mechanics and the theory of general relativity—the theory that says space and time curve in the presence of mass. By extremely cooling a small mechanical oscillator and then manipulate it, physicists try to place an object at two positions simultaneously. In that scenario it is unclear which mass distribution determines the curvature of space-time. This gives rise to a new kind of uncertainty and might hold a clue to a large mystery in physics: the collapse of a wave function. Experimental research on this theory could lead to a groundbreaking result.

Cooling
It is safe to say that there are enough reasons for Oosterkamp to create an ultra-cold environment. And indeed there should be, as it is a giant tour de force to go from room temperature to one milliKelvin. First, they set the fridge to four Kelvin. For this the research team uses a compressor that alternately makes helium (4He) compress and expand in a small chamber. The resulting low temperature is the starting point to let a container of 4He condense inside a second cooling system. They add the helium isotope 3He to this container, producing a mixture of 3He-4He. This requires mixing enthalpy, so it extracts energy from its surroundings. Next, a pump separates the 3He atoms from their heavier counterparts, after which the researchers re-use them in the same process.

Isolation
In the meantime, it is of vital importance for the setup to stay as well isolated as possible. Oosterkamp’s group uses vacuum layers, heat shields and many ultra-thin layers of plastic and aluminum foil. Now in order to push through to truly extreme temperatures, they take a piece of metal and cool it down inside a magnetic field of two Tesla. A magnetic field creates order and therefore low entropy, because it makes all the metal’s nuclear magnets point in the same direction. When they slowly turn off the field, the nuclear magnets start to point in random directions again. This chaos increases the entropy, which cools the system even further. ‘In the end we reach 1 mK,’ says Oosterkamp. ‘It’s no world record, but that’s not why we do it. It is about the research we’re able to conduct at that temperature, and that is booming once again. Twenty years ago you saw the people at conferences on this area growing older and older, but now we have a new generation of young scientists studying old cooling techniques. With all the new research questions in quantum mechanics, science moves back to ultra-low temperatures. And for this Leiden is both historically and currently the place to be.’

Four quantum physicists from Leiden and Delft addressed a full room of 200 people on entanglement at a mini symposium of the Royal Dutch Academy of Sciences (KNAW). Carlo Beenakker and Dirk Bouwmeester took the stage on behalf of Leiden More info

University and Ronald Hanson and Stephanie Wehner represented TU Delft. Together they explained the ‘ghost-like’ existence of entanglement between elementary particles, and how we can make use of this bizarre phenomenon.

Erwin Schrödinger’s famous thought experiment shows how strange the laws of quantum mechanics are. Beenakker opens the symposium with the most talked about cat in history. Elementary particles are never in one place at a time. They occupy multiple positions simultaneously, according to a probability distribution that reflects the odds of where the particle will report at the moment we look at it. Because only when we look, we force the particle to take a seat. As if the music stops at musical chairs. This uncertainty goes not only for position, but also for properties like spin, polarization and decay. In his mind, Schrödinger put his cat in a box, together with a mechanism that either breaks a bottle of poison or leaves it intact, depending on the quantum state of one atom. As long as he doesn’t look inside the box, the quantum state is ambiguous, and the cat is both dead and alive!

Entanglement

In an absurdist way, the fate of Schrödinger’s cat is intertwined with the particle’s state. In this example we still see a logical link between poison and death, but we could also connect two elementary light particles so that the one state depends on the other. If you would measure a vertical polarization for one photon, it is automatically determined that the other is horizontally polarized. Scientists call this entanglement. Or in the cynical words of Albert Einstein: ‘spooky action at a distance’. The weird thing is that the second photon’s fate is instantly determined by measurement of the first photon. Information would then travel faster than light!

Bell test

Last year, Einstein paid a price for his historic statement when Ronald Hanson made the front pages by conclusively proving that entanglement indeed exists. Hanson explains his so-called Bell test as a game played by Alice and Bob. Two questions are possible—‘Which glass of wine?’ and ‘Which bottle of wine?’—and two answers—‘red wine’ and ‘white wine’. If both are asked about the bottle, they will score points if they answer the same. In any other case they should answer differently. Mathematicians know that the best possible joint strategy will lead to a score of no more than 75%. Hanson had two entangled particles play a similar game, and those managed to obtain a miraculous score of more than 75%. Einstein suspected that quantum states are predetermined in some way, even though we don’t see it. Hanson proved this sumption wrong. ‘Two tiny particles beat the most powerful supercomputers,’ says Hanson. ‘That truly shows the power of quantum.’

Encryption

Another example of quantum entanglement’s promise is uncrackable encryption, on which Dirk Bouwmeester elaborates with many formulas. Those are, no matter how tedious, indispensable when talking quantum mechanics. Beenakker agrees: ‘In my classes I never talk about interpretations, but I use formulas. That is the modern way.’ In the end, mathematics is the only language in which the theory can be truly explained. Sometimes this leads to misunderstandings, as Bouwmeester experienced while working in London. ‘We were working on quantum encryption when three men from the secret service MI5 paid us a visit, in long black coats. I explained that it truly is perfectly safe. They responded almost simultaneously: “that is for us to decide”.’

Teleportation

The promise that perhaps speaks most to the imagination is teleportation. In theory you could teleport a human body to Australia in literally zero seconds; modern faxing in a sense. Matter itself doesn’t travel, but the information on quantum states is copied into different atoms. Whether consciousness is copied along is still fuel for philosophical debate. Yet, quantum mechanics has evolved from a philosophical problem in the previous century to a technological challenge in this century, according to Beenakker. We are on the threshold of an era of quantum technology.

Ask any Dutch person how they first got engaged with science, and the answer will probably be: Het Klokhuis. In fact, chances are that many of the Dutch researchers at our institute got their first inspiration to become a scientist More info

from this kids tv show, which has been running for three decades on national television. And we can even say with certainty that some of today’s audience will end up conducting groundbreaking research at LION, some twenty years from now. Without them knowing, they got acquainted with their future employer during an episode on cold temperatures, as Het Klokhuis paid the Leiden Institute of Physics a visit.

firms in our economy. Examples of such relationships include ownership ties (firm A owns firms B) or board interlock ties (linking firms based on shared senior level directors). This talk considers the global corporate network, demonstrating results of different studies in which the connectedness of the largest 1 million firms across the globe is investigated. Topics include data quality implications, network topology, centrality analysis and community detection. The results provide interesting insights in the world's most powerful countries and firms, as well as patterns illustrating the network structure of tax heavens.

Leiden biophysicists have found a new possible way to prevent metastasis. They have located ‘sinkholes’ on cancer cells where receptor proteins disappear from the surface. If a drug could push these proteins towards those areas, it would prevent the cells More info

In order to find a treatment for cancer, scientists need to do fundamental research to understand the inner workings of cancer cells. Then they can identify any weak spots and ask pharmaceutical researchers to find a drug that attacks the cells at their Achilles heel. For this reason, Professor Thomas Schmidt and his group studied receptors called CXCR4 on a bone cancer cell. Those receptors perform their job on their host cell’s surface; directing the cell towards a specific place inside the human body. Metastatic cells need such receptors on their surface, otherwise they won’t know where to go and stay at the original tumor.

Adaptation mechanism
When CXCR4 proteins receive a signal from some part of our body, they become activated and lead their host cell in the right direction. If a strong signal activates those receptors, cells use a mechanism that removes receptors from their surface, so they don’t become too sensitive. A similar adaptation happens when your pupil constricts if you look at the sun. Also your ears work this way; we perceive sounds that are ten times louder than street noise only as two times as loud.

Sinkholes
Now lead author Elena Beletkaia and her colleagues have located ‘sinkhole’ areas on the bone cancer cell’s surface. Here the CXCR4 receptors sink down into the inner cell, or in biophysicists’ terms, get immobilized. ‘Doctors find that if patients have a high number of CXCR4, it is a bad prognosis,’ says Schmidt. ‘One of their ideas is to develop a drug against CXCR4, to silence them. But now that we know where the receptors disappear from the surface, there could be a different kind of intervention: push the receptors into the sinkhole, and then they are gone.’

Fluorescence microscopy
The researchers used epi-fluorescence microscopy to study their samples. This technique enables them to see individual molecules by highlighting a few of them each time they take a picture. For every snapshot they make different molecules fluorescent, causing them to emit light at a specific wavelength. This is easily filtered out from the indistinguishable noise from the many other molecules. They keep repeating this process until most molecules are located. Using this technique, the group was able to trace CXCR4 receptors and spot the location of their sinkhole hide-out.

Do you think you can pitch your research to a general audience in 3 minutes? Then sign up for the FameLab 2016 Dutch preliminaries! Anyone between 21 and 40 years old and working in or studying science can participate in More info

one of the biggest science communication competitions in the world. And did we mention that slideshows are not allowed? It truly comes down to giving the best elevator pitch you have in you!

FameLab is an international competition organized by the British Council with finals in the UK at the Cheltenham Science Festival and national rounds in many countries, including The Netherlands. The Leiden preliminaries are on February 16 (register before January 29) and the national finals are on April 22 in Utrecht. Here is a full overview of the Dutch preliminaries:

Groningen: February 11 (register before January 31)
Utrecht: February 11 and 18 (registration closed)
Leiden: February 16 (register before January 29)
Nijmegen: February 17 (register before January 25)
Amsterdam: February 26 (register before January 14)
Wagingingen: March 4 (register before January 15)

The Koninklijke Nederlandse Akademie van Wetenschappen organizes a mini symposium on Wednesday January 20 in Amsterdam, titled 'What is the importance of quantum entanglement?' Among the speakers are LION's Carlo Beenakker and Dirk Bouwmeester.

Biophysicist Rolf Harkes has developed a microscope to optically localize individual molecules in living cells. It improves monitoring of diseases like cancer and Parkinson’s at the cellular level. He successfully defended his PhD thesis on this subject on January 13th More info

in Leiden.

For some decades it has been possible to zoom in on the atomic level by using an electron microscope. However, this technique is not applicable on organisms, since the radiation is harmful to living cells. To study biological matter with ultra-high precision, scientists need a microscope that uses ordinary light. Optical microscopy faces limitations on the small scales however, because Heisenberg’s uncertainty principle tells us that we cannot measure both location and angle of an incoming photon with absolute certainty. So light originating from a point will be imaged as a hazy spot rather than a point. If molecules are stacked too closely together, which is usually the case, a regular optical microscope cannot distinguish them from each other because their images overlap.

Fluorescence
A new technique called Single-Molecule Localization Microscopy (SMLM) tackles this problem. It is based on a technology pioneered by Leiden physicist Michel Orrit. Scientists randomly make some individual molecules fluorescent. For a short time those will emit light at a distinct wavelength, making it easy to filter out their signal from the noise of the many other molecules. Because this gives an image of only a few molecules, there are no overlapping localizations. This process is then repeated, with other molecules made fluorescent, until they have all been located.

Keeping score
Harkes, supervised by Prof. Thomas Schmidt, developed an SMLM microscope to show its promise for life cell research exemplified for a protein linked to Parkinson’s disease and created new analysis methods for broader use, like a counting method. ‘You would think that you can easily keep score of the number of molecules by counting the times you localize one,’ he explains. ‘It turns out that some are localized multiple times. And this doesn’t happen on the exact same location, as there is always still some uncertainty. So I developed a mathematical analysis that keeps track of localizations nearby a previous one, and corrects for double counts.’

Study Parkinson’s
Harkes’ technology and analysis methods help visualize biological structures in ultra-high resolution, down to the molecular level. This enables medical professionals to better monitor progression of tumors and researchers to study the mechanisms behind diseases like Parkinson’s. Harkes: ‘I used SMLM for example to look at the spatial distribution of the protein α-synuclein in cells. That is a good example of SMLM’s promise to study diseases, like Parkinson’s in this case.’

Professor Carlo Beenakker has been awarded a FOM Projectruimte subsidy to build a theoretical model of a majorana gun—a very promising instrument for quantum computers.

Regular computers store information in bits, which represent a 0 or a 1. Quantum computers More info

use qubits. These information units are a superposition of 0 and 1. They represent simultaneously to a certain degree 0, to a certain degree 1 and in two ways to a certain degree both digits together. In this way, qubits possess information about four parameters instead of two. Multiple qubits therefore make a computer exponentially more powerful compared to a system based on ordinary bits.

Control
It is however still an enormous technological challenge to build a quantum computer. You need individual elementary particles as building blocks, rather than macroscopic magnets in a hard drive. And particles are not some tangible balls that you can pick up, but elusive mathematical wave functions. This makes them extremely hard to control and manipulate with machines from our macroscopic world. After enduring research, scientists know how to manage this for electrons and photons, but it remains a mystery how this would work with the building blocks for quantum computers; so-called majorana particles.

Majorana gun
To give this technology’s development a push, Beenakker uses his FOM grant to develop a theoretical model for a majorana gun. Once built, such a device is able to fire individual majorana particles. It is the superconducting version of an electron gun. In a superconducting material, electric current is not made up of everyday electrons, but so-called Cooper pairs—superpositions of two electrons. If you dissect them, you are not simply left with two electrons, but with particles of positive, negative or neutral charge. In the latter case you would have a majorana particle.

Vision
‘A majorana gun can be very useful for a quantum computer,’ Beenakker says. ‘That technology is a vision for me that provides direction in my fundamental research. I want to create a theoretical model for a gun that injects a majorana pair on demand. I don’t have a lab, so we work on the underlying theory. This means that we write computer code for simulations and think about designs.’

The FOM grant gives Beenakker financial room to hire a PhD student and a postdoc.

We are halfway through the academic year 2015-2016 and LION is off to a great start with many beautiful discoveries. With the year 2015 coming to an end, it is time to take a break and enjoy the things we've More info

With many more papers published, prizes won, grants awarded and colloquiums organized we can safely congratulate all LION members with a great first half year of 2015-2016, and wish everyone within and outside the institute happy holidays! We'll be back in 2016 with more exciting discoveries.

Normally, you would want your jewelry to be as durable as possible. But Leiden physicists break gold and platinum chains on purpose. By doing so, they have found evidence for forces that could be used for driving tiny nanomotors, just More info

Experimental physicist Jan van Ruitenbeek and his group take extremely small metal chains consisting of only a few single atoms in a row and send an electric current through. When the chain breaks, they repeat the process, up to thousands of times. In most cases it breaks in a similar way as the filament of a light bulb that has burnt out: the electric current heats the chain until it becomes too hot and falls apart.

The weird phenomenon is that some of the chains break at a surprisingly low voltage. This was already measured more than ten years ago, but could not be explained. Now, first author Carlos Sabater thinks he found the reason; a combination of two forces acts on the chains, the so-called wind force and the Berry force. Together they cause the chain beads to move and rotate so much that the bond between them breaks.

These forces resemble a river moving a water wheel. Therefore, they could be of great use for driving rotating parts in nanomotors. These have a wide range of possible applications and could for example be used for drug delivery in the body. Conventional nanomotors only work if they are driven by a cyclic force, which makes things very complicated. However, nanomotors based on the Berry and wind force only need a simple electric direct current. A stream of electrons continuously pumps energy into the metal atoms via the wind force, while the Berry force causes them to rotate.

Until now, the Berry force has never been demonstrated experimentally. The direct observation of a rotating nanowheel is very challenging, but the breaking atom chains are an indirect proof for it. The now published results match very precisely with theoretical predictions. The researchers continue to work on more direct ways of monitoring the force.

On an early morning in the depths of the Mediterranean Sea, a group of technicians installed the first detection unit of a large under-water neutrino telescope: KM3NeT. Leiden physicist Dorothea Samtleben is part of the collaboration of scientists and engineers More info

that make this enormous project happen. Once completed, it will be largest neutrino telescope in the Northern Hemisphere and will be used to study the fundamental properties of neutrinos and map the high-energy cosmic neutrinos emanating from extreme cataclysmic events in the Universe. The Leiden Institute of Physics played already a prominent role in the data analysis of earlier smaller prototypes and the testing of the current detector.

'KM3NeT will start a new era in this field' says Samtleben 'With its unprecedented size and excellent resolution of the neutrino directions it will provide for rich new information to explore our Universe.'

Neutrinos are the most elusive of elementary particles and their detection requires the instrumentation of enormous volumes: the KM3NeT neutrino telescope will occupy more than a cubic kilometre of seawater. It comprises a network of several hundred vertical detection strings, anchored to the seabed and kept taut by a submerged buoy. Each string hosts 18 light sensor modules equally spaced along its length. In the darkness of the abyss, the sensor modules register the faint flashes of Cherenkov light that signal the interaction of neutrinos with the seawater surrounding the telescope. The detector is connected by a 100 km cable to the shore station located in Portopalo di Capo Passero in the south of Sicily.

Samtleben: ‘Of course we had tested the deployment procedure beforehand and also all components of the detector. But this was the first time we installed a full real detector unit, so we waited anxiously for the signal chain to become operational. We followed the news on a live feed over the deployment day. In the evening we could finally evaluate the first data, proving the full functionality. We celebrated the next day at Nikhef (Institute for Subatomic Physics) where a major part of the construction of this detection unit took place.'

Maarten de Jong, director of KM3NeT and professor at Leiden University, is excited as well: ‘This important step in the verification of the design and the technology will allow the KM3NeT Collaboration to proceed with confidence toward the mass production of detection strings and their installation in the Mediterranean Sea’.

A building block of KM3NeT comprises 115 detections strings; the full detector has many building blocks with a total of a few hundred strings (Credit: Marc de Boer/Ori Ginale)

The neutrino detection string wound around the spherical deployment frame is hoisted down into the Mediterranean Sea.

View from the seabed of the Mediterranean Sea as the first detector of the KM3NeT neutrino telescope is being installed.

Have you ever wanted to play the villain in a movie and create your own virus? It’s easy: combine one molecule of genomic nucleic acid, either DNA or RNA, and a handful of proteins, shake, and in a fraction of More info

a second you’ll have a fully-formed virus.

While that may sound like a bad infomercial, in many cases making a virus really is that simple. Viruses such as influenza spread so effectively, and as a result can be so deadly to their hosts, because of their ability to spontaneously self-assemble in large numbers.

If researchers can understand how viruses assemble, they may be able to design drugs that prevent viruses from forming in the first place. Unfortunately, how exactly viruses self-assemble has long remained a mystery because it happens very quickly and at very small length-scales.
A system is needed to track nanometer-sized viruses at sub-millisecond time scales. And now, lead author Sanli Faez from the Leiden Institute of Physics has developed such a technique in collaboration with researchers at Harvard University, MIT, the Leibniz Institute of Photonic Technology, the University of Jena, and Heraeus Quarzglas, a manufacturer of fiber optics. The team, led by Vinothan Manoharan (Harvard University), published their paper in ACS Nano.

There are two main challenges to tracking virus assembly: speed and size. While fluorescent microscopy can detect single proteins, the fluorescent chemical compound that emits photons does so at a rate too slow to capture the assembly process. It’s like trying to observe the mechanics of a hummingbird’s flapping wing with stop-motion camera; it captures pieces of the process but the crucial frames are missing.

Very small particles, like capsid proteins, can be observed by how they scatter light. This technique, known as elastic scattering, emits an unlimited number of photons at a time, solving the problem of speed. However, the photons also interact with dust particles, reflected light, and imperfections in the optical path, all of which obscure the small particles being tracked.

To solve these problems, the team decided to leverage the outstanding quality of optical fibers, perfected over years of research in the telecommunication industry. They designed a new optical fiber with a nano-scale channel, smaller than the wavelength of light, running along the inside of its silica core. This channel is filled with a liquid containing nanoparticles, so that when light is guided through the fiber’s core, it scatters off the nanoparticles in the channel and is collected by a microscope above the fiber. The researchers observed the motion of viruses measuring 26 nanometers in diameter at a rate of thousands of measurements per second.

‘The main goal of our research has been to develop a general tracking platform based on elastic scattering to overcome the limitations of fluorescence microscopy,’ says Faez. ‘We worked with the virus as a notable demonstration of the advantages of using this platform. Apart from virus research, we can now use it for many different applications, for instance counting vesicles in the bodily fluids, which is too expensive for regular screening with existing methods based on electron microscopy. Cancer cells produce more vesicles than healthy cells, so if we can monitor someone’s vesicle count accurately enough, we can possibly develop a new diagnostic tool for cancer.’

Α-synuclein, a protein associated with Parkinson’s disease, proves to bind with membranes in a surprisingly efficient way. It confirms scientists’ suspicion of the protein’s leading role in the transmission of neurotransmitters between nerve cells in the brain.

Physicists of the universities of Leiden and Twente studied the binding process of α-synuclein with small vesicles. During the research, these played the part of specific vesicles which attach to the brain membrane and transfer neurotransmitters. First author Pravin Kumar and his colleagues used the Leiden developed method of keeping track of the binding process through electron spins. Their research is published in PLoS ONE.

Parkinson
Parksinson’s disease is the second most prevalent neurodegenerative disorder. Within the brain of a Parkinson patient, something goes wrong during the transport of neurotransmitters, which provide the communication between nerve cells. But it is still unknown when and where exactly the error occurs. However, scientist have already discovered that proteins accumulate in the brain, and α-synuclein is notably abundant there. The suspiciously dominant presence of this protein was cause for group leader Dr. Martina Huber to study its behavior.

Tracing technique
Her group studied the binding of α-synuclein with vesicles, using their electron paramagnetische resonantie (EPR) technique. They placed several labels on the molecule, and by manipulating their electron spins they could keep track of the labels’ positions during the process. In this way, they monitored the behavior of the protein. They measured the mobility of the labels to determine which parts of the protein have maximum binding, and therefore move the least.

Surprise
‘Much to my amazement it turns out that α-synuclein binds surprisingly well,’ says Huber. ‘Normally speaking, this kind of vesicles binds poorly because it is not very negatively charged. Still, we see this result.’ It provides a piece of the puzzle to figure out the cause of Parkinson’s disease. Huber: ‘The occurrence of a disease has many factors, including the truly malicious processes, but also correction mechanisms of the body that work correctly or fail. We need to map out all those pieces of the puzzle to see where exactly things take a turn for the worse. Now we see that α-synuclein plays an important role in Parkinson’s. You have a much better chance of finding a treatment once you understand how a disease works, so it is important that we came a step closer towards getting the full picture.’

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Article Parkinson’s Protein α-Synuclein Binds Efficiently and with a Novel Conformation to Two Natural Membrane Mimics
Pravin Kumar, Ine M. J. Segers-Nolten, Nathalie Schilderink, Vinod Subramaniam, Martina Huber
DOI: 10.1371/journal.pone.0142795
The protein α-synuclein (green) is plastered with labels on several sites. By manipulating the labels’ electron spins (red), it becomes clear with which site and how strongly the protein binds with the membrane.
Neurotransmitters (purple balls) are trapped inside vesicles (brown circles). During the transfer of neurotransmitters, the vesicle attaches to the brain membrane (brown edge around grey area), so that an opening emerges towards the synapsis (white area between nerve cell below and nerve cell on top). In the brain of patients with Parkinson’s disease something goes wrong in this process, and α-synuclein turns out to play a major role in this. The protein proves to bind strongly with the concerning vesicles, which boosts the suspicion that they have something to do with the disease. Photo Credit: Gayla S. Keesee

The long hallways of Leiden’s science faculty are quiet and peaceful when suddenly a young student flashes by. A few moments later, he skims along a girl holding a wooden plate plastered with tin cans, copper wires and duct tape. More info

It’s a scene that would fit perfectly in a sci-fi movie like Back to the Future, but in reality we are witnessing two physics students testing out their own radar gun, developed for the Bachelor course Practicum Physics Experiments. It gives participants of today’s Experience Day a taste of what to expect from a Physics education in Leiden. Leiden University organizes this biannual day for high school students to familiarize themselves with a study of their interest.

Bachelor Physics students Ellen Riefel and Mark Knigge needed to come up with a device to build for their practicum course, when they came across radar gun building instructions from MIT. It seems like an extensive task, but Ellen and Mark decided to take on the challenge and start devising a plan to actually build one. ‘At first, I didn’t expect it to really work,’ says Mark. ‘But in the end we succeeded and the radar gun performs even better than we hoped for.’ The two students taped a set of electronics on nothing more than a wooden plate and turned it into a working radar gun, with an error margin of only a few km/h.

From one side of the plate, a tin can sends out a 3 GHz radio signal, which bounces off a moving object and is collected by a similar can on the other side of the device. A small box transforming the analog signal to a digital one sends the information to a laptop. Ellen and Mark wrote an algorithm so the laptop can calculate the speed from the measured change in frequency induced by the so-called Doppler effect. Mark: ‘We were puzzled for a long time by an unexpected output. In the end we figured out that we had forgotten about a factor 2, because the signal travels back and forth to the car. In a project with so many elements these are the things you typically overlook.’ Ellen: ‘It is frustrating when your code doesn’t work, but once it finally comes together, it is really satisfying.’

Armed with a working radar gun, the team went outside to test their device on the streets. After three seconds aimed at a car, it produces a reliable signal, much to the fear of some drivers. ‘We saw cars braking and switching lanes and back,’ says Ellen. And while the gun cannot be used for actual ticketing, it is very useful in physics outreach. Mark: ‘We use it as an example of how exciting Physics can be. We showed it for example at the Physics Ladies Day a few weeks ago.’

Physicists from Leiden University and the FOM Foundation have found a way to better understand the properties of manmade 'smart' materials. Their method reveals how stacked layers in such a material work together to bring the material to a higher More info

level. Group leader Sense Jan van der Molen and his research team will publish their results today (26 November 2015) in Nature Communications.

Can we design smart materials with entirely new properties? A highly promising way of doing this is to stack extremely thin layers – just one atom thick – into a three-dimensional material; a sort of sandwich cake. Interestingly enough, the properties of these composite materials are not only determined by the properties of the individual layers. The interaction between the layers also plays a significant role. Consequently, such a layered material can have very different properties than you might expect based on the combination of properties of the individual layers; the whole is more than the sum of the parts. Physicists from Leiden University and FOM have developed a technique that allows them to study the interaction between the material layers.

Band structure
The electronic properties of a material, expressed in what is called the band structure, determine how the material behaves. The band structure tells you what energy an electron in the material can have and how this energy depends on the velocity of the electron. There are allowed energies (‘bands’) and forbidden energies (‘band gaps’). A large part of this band structure was previously difficult to measure. First author Johannes Jobst and his colleagues overcame the problem by using and upgrading a special microscope: a Low-Energy Electron Microscope (LEEM).

The microscope fires electrons with a specific energy at the probed material. Researchers subsequently measure how many electrons of various energies are reflected. When an incoming electron encounters an unoccupied state in the material, it is not reflected. Conversely, when there are no free states with the energy of the incoming electron, the reflection rate is high. Using this method, the researchers can measure which occupied and unoccupied electron states are present in the layered material and consequently what the band structure looks like.

By doing this with various stacks of graphene, the researchers managed to reveal how the bands associated with the different layers interact with each other. The method has a 100,000 times higher spatial resolution than conventional methods. This is important because the current layered materials have an extremely small surface (far smaller than a square millimetre).

Designer materials
As soon as scientists have a good understanding of the interaction, they might be able to take the next step: 'We want to be able to choose certain properties in advance and subsequently stack the layers in such a way to realise the desired material,' says Sense Jan van der Molen. 'Such designer materials are the long-term objective.'

a) The new measurement technique fires electrons at the stacked materials at an angle. By analysing the reflection of the electrons, researchers can better understand how the two dimensional layers work together to establish the properties of the combined material.
b) The long term goal is for researchers to design new materials, by building a 'sandwich cake' of material layers with the exact desired properties.

Tomorrow (November 26th) at 16:35, science tv show De Kennis van Nu Nieuws shows the painting of the first of many physics formulas on walls in Leiden’s city centre, on channel NPO 2. This is a project More info

of LION’s Sense Jan van der Molen and Nikhef’s Ivo van Vulpen, who want to give more visibility to Leiden’s scientific history, alongside its rich history in humanities, represented by the omnipresent wall poems. Yesterday, prof. Robbert Dijkgraaf unveiled Einstein’s field equation on an outside wall of Museum Boerhaave. We celebrate the 100th birthday of the corresponding theory of general relativity this week, making it the perfect formula to kick-off the project. The so-called cosmological constant in the equation was added later, after discussions with Leiden’s Willem de Sitter.

Abstract
We provide an introduction into the research program on bibliometric network analysis at Leiden University’s Centre for Science and Technology Studies (CWTS). We demonstrate two popular software tools for bibliometric network analysis developed at CWTS: VOSviewer and CitNetExplorer. We also discuss the techniques that we have developed for network layout and community detection. Finally, we use bibliometric network analysis to study the field of network science and the contributions made to this field by researchers at Leiden University.

On Tuesday November 24th, professor Robbert Dijkgraaf will unveil a painted version of Einstein’s Field Equation on Museum Boerhaave’s outside wall. This launches a series of at least ten Physics formulas to be visualized on walls throughout the historic city More info

center. All will have a direct connection to Leiden science. Einstein coined his equation exactly 100 years ago as the mathematical foundation for his theory of general relativity. The so-called cosmological constant was added later, after discussions with Leiden’s Willem de Sitter.

Physicists Sense Jan van der Molen (LION) and Ivo van Vulpen (NIKHEF) initiated the idea to put Leiden’s scientific history in the spotlights, next to its rich history in the humanities, already emphasized by the omnipresent wall poems. The city has an enormous scientific heritage and is the birth ground of many famous and widely used physical formulas. In addition to the Einstein Field Equation, the Leiden people will e.g. be inspired by Snell’s law (refraction of light), Huygens’ pendulum law, the Lorentz contraction formula (theory of special relativity), Van der Waals’ equation of state (thermodynamics), the Ehrenfest Theorem (Quantum Mechanics), the Oort constants (Oort cloud) and the electron spin formulas.

All formulas will be designed and painted by Ben Walenkamp and Jan Willem Bruins of Stichting TEGEN-BEELD, who already enriched the city with over a hundred wall poems.

The project is supported by Leiden municipality, Leiden University’s Science Faculty, the Gorterstichting and the Lorentzfonds.

What: Unveiling Einstein wall formula by Robbert Dijkgraaf

When: Tuesday November 24th at 15:45

Where: Museum Boerhaave, Lange St. Agnietenstraat 10, Leiden

More information: This email address is being protected from spambots. You need JavaScript enabled to view it. (Sense Jan van der Molen), This email address is being protected from spambots. You need JavaScript enabled to view it.
and This email address is being protected from spambots. You need JavaScript enabled to view it.

2013. They are challenged to submit a good, coherent, fluently written article of 2,000 words on their Physics research (dissertation).

A jury, consisting of members of the editorial board of the journal, will prepare a proposal for the three best submissions. The editorial board will ultimately decide who the winners will be. The three best submissions will be published in the Dutch Journal of Physics (NTvN). Additionally, the winning authors will obtain a cash prize: first prize 1000 euro, second prize 750 euro and third prize 500 euro. The prizes will be awarded on 8 April during FYSICA 2016 in Nijmegen, the annual meeting of the NNV.

The manuscript must be submitted to This email address is being protected from spambots. You need JavaScript enabled to view it. before December 1, 2015.

member of Luca Giomi’s group at the Leiden physics institute, developed a model to help biophysicists analyze experimental data on DNA research.

Scientists study mechanical properties of DNA strands by stretching them. To zoom in on the actual genetic code—the base letters—biophysicists fluorescently label letters so they can trace them. However, these labels influence the mechanical properties of the DNA strand. During his master’s at the Eindhoven University of Technology, Schakenraad designed an analytical model that corrects for any influence of the labels.

After obtaining his master’s degree, he received a Leiden/Huygens fellowship to continue his work on bio-mechanics in Leiden and carry out theoretical research on cell morphology. ‘I got a phone call from the chairman of KHMW to let me know I won,’ says an excited Schakenraad. ‘I knew that my previous supervisor had nominated me, but I had no idea if I’d have a chance to win.’

To celebrate the 100th anniversary of Einstein's Theory of General Relativity, there is a special colloquium this Monday November 16th at 17:30:
"100 Years of General Relativity - From Einstein in 1915 to Relativistic Cosmology in the 21st Centrury", by Prof. More info

Pedro Ferreira (Oxford University).
Pedro Ferreira is author of the popular scientific book The Perfect Theory. Before the colloquium there is a borrel, which starts already at 16:30.

Physical Cosmology is the success story of modern physics. Observations of the large scale structure of the universe
have allowed us to characterize the cosmological model with unprecedented precision. What is less appreciated is that we have
learnt a great deal about the building blocks of Relativistic Cosmology: about space-time, it origins and the role of general
relativity in the evolution of the universe. In this talk I will show how, over the last two decades, we have made tremendous progress in
characterizing the overall metric of space-time- its homogeneity and isotropy- as well as its properties at very early times. I will show how
exploring general relativity can play a crucial role in solving one of the big cosmological puzzles (the dark energy problem) and how
cosmology can be used to find new, precision constraints on general relativity. I will discuss a vision of the future in which we attempt
to completely map out the large scale characteristics of space-time using innovative techniques and future surveys.

Tonight, Thu Nov 12th, over sixty participants take part in a meeting at the science faculty in which high school teachers deepen their knowledge of physics and astronomy and explore new teaching methods. The Physics and Astronomy departments organize these More info

teacher meetings three times per year.

Research group leader Sense Jan van der Molen opens the evening with a lecture on the electrical properties of two-dimensional materials, in which he elaborates on his research with low-energy electron microscopy. After the dinner break, Pedro Russo will introduce the European education project TEMI—Teaching Enquiry with Mysteries Incorporated—that uses the concept of mysteries to stimulate enquiry-based learning in high school education. He is followed by Wouter Spaan, who presents his book Show de Fysica. Prof. Dirk van Delft, director of museum Boerhaave, will talk about the exhibition Einstein & Friends that is currently showing. Henk Buisman, who coordinates the connection between high school and university, wraps up the meeting by providing insights into the recent developments in setting up a series of class experiments that fit the new physics high school curriculum.

LION's first Image Award has revealed the artistic side of many physicists. Although the judges could only select three images for their top 3, there were lots of other beautiful pieces of art. It would be an awful waste of More info

The physics of black holes appears to be as far removed from the physics of electrons in metals as it can be. Now Prof. Jan Zaanen, Prof. Koenraad Schalm (both Leiden University), Dr. Yan Liu and Dr. Ya-Wen Sun (both More info

Madrid University) have written a book presenting how the one can be used to explain exotic behavior of the other with the new mathematical techniques of holographic duality. Published by Cambridge University Press, it is as of now available at Cambridge University Press or Amazon.

‘This is the first book dealing with a surprising and major development at the very frontier of theoretical physics revolving around the application of mathematical techniques originating in string theory to condensed matter physics,’ say Zaanen and Schalm. ‘This is a rapidly developing research frontier, where literally hundreds of theoretical physicists are actively contributing. It was born as recently as 2007, and since then it has developed into a flourishing field. The rate of progress has been staggering and there is a shared perception that this reflects a major breakthrough at the very heart of modern physics.’

Abstract

A pioneering treatise presenting how the new mathematical techniques of holographic duality unify seemingly unrelated fields of physics. This innovative development morphs quantum field theory, general relativity and the renormalisation group into a single computational framework and this book is the first to bring together a wide range of research in this rapidly developing field. Set within the context of condensed matter physics and using boxes highlighting the specific techniques required, it examines the holographic description of thermal properties of matter, Fermi liquids and superconductors, and hitherto unknown forms of macroscopically entangled quantum matter in terms of general relativity, stars and black holes. Showing that holographic duality can succeed where classic mathematical approaches fail, this text provides a thorough overview of this major breakthrough at the heart of modern physics. The inclusion of extensive introductory material using non-technical language and online Mathematica notebooks ensures the appeal to students and researchers alike.

Over 90 female future students from VWO 5 and 6 explored the Physics education in Leiden during the annual Physics Ladies’ Day, last Friday. The girls went speed dating with female physicists from several companies and the university. Students told More info

about their choice for Physics, what it entails and student life. The participants also carried out some experiments themselves in the laboratory. ‘What courses do Physics students take?’ and ‘What are the career options with a Physics degree?’. These and many other questions were answered by LION students, professors and alumni.

Adeline and Susanne (5 VWO)

This was also the reason for Adeline and Susanne to register for the Physics Ladies’ Day. ‘We already know we want to do a science study, but we’re not sure yet which one. This is a good moment to look around.’ Adeline: ‘Today I found out that Physics is more than just formulas.’ Susanne: ‘Also for me it became much clearer what the Physics education exactly entails.’

Hannah and Francesca (5 VWO)

Hannah and Francesca are orientating on a their future studies as well. Hannah: ‘I’m still in doubt between Medicine and Physics. Today I saw during in a lecture that you can combine Physics with other disciplines, like Biology. The education is very broad.’ Francesca: ‘During the speed date sessions, I noticed that you can go in many directions with a Physics degree. I spoke for example with someone from Booking.com. I didn’t expect that!

Nynke (6 VWO)

Nynke: ‘At lunch I talked with one of the students. She combines the studies Astronomy, Physics and Mathematics. I visited their presentations during the Open Day. It’s good to know that you can combine them, perhaps I don’t need to choose between different studies.

Addition to Open Days and Proefstudeerdagen

The Physics Ladies’ Day is an addition to Leiden University’s Open Days and Proefstudeerdagen. VHTO, the national expertise organization for girls/women in science, organizes in collaboration with education institutes and companies many activities for girls to increase the participation of girls and women in the world of science, technology and ICT.

Tomorrow, LION organizes its annual Physics Ladies Day for female high school students. To mark this festive day, we put the spotlights on three female researchers, who talk about their experiences in physics.

‘I have been fascinated with physics since I first started to study it in middle school.
What attracted me most was the possibility of framing the laws of nature in a rigorous way and then tackle real problems. I really enjoyed the problem solving challenge!

‘My favorite subject was theoretical physics, in particular field theory and particle theory. It kind of came to me naturally to continue my studies in physics and that eventually turned into a career. Soon after finishing my MSc in Theoretical Physics, I moved to the US to pursue my PhD, at Syracuse University, and then a long postdoc at MIT (Boston). The international experience has been of paramount importance for me. It widened my scientific horizons and helped me grow into a well-rounded individual.

‘I am a theoretical cosmologist, so I study the Universe, how it started and evolved into the structure that we observe around us. The evolution of the Universe spans an impressively wide range of energies and scales, so it really offers us an invaluable window into gravity and fundamental physics.’

Gesa Welker, PhD student

‘A problem seems to be unsolvable, but with a little trick you can suddenly solve it after all. Besides a deep fascination for nature, these moments brought me to go study physics. I found it a challenging study, but precisely that is what makes it so much fun. I learned about the mechanisms behind nature, for example why the sky is blue or how the Sun produces light through nuclear fusion. And I still find it astonishing that we can exactly describe those phenomena with mathematical formulas.

‘Science is very internationally oriented. I did my studies in Germany (I’m German myself), after which I got the opportunity to study in the US for a year. One and a half year ago I moved to The Netherlands to start my PhD research in Leiden. I work together with people from many different countries in my lab. It is kind of crazy that my love for natural sciences has brought me in touch with different cultures and languages. Two years ago, I would have never guessed that I’d ever learn Dutch, and now I speak it every day. Fortunately, the laws of quantum mechanics are the same in every language, because that is what my research is about. According to those laws, it is for instance possible that an object at the same time moves and doesn’t move. I try to get a tiny mirror in that state. That isn’t easy, but I believe I will succeed eventually.’

Vera Meester, PhD student

‘I think natural sciences are one of the coolest things there are. With the knowledge you obtain during your physics studies you learn how the world works and how you become a discoverer yourself. I developed a great enthusiasm for physics research during my Bachelor’s. I work with particles so tiny that they are invisible to the naked eye. Particles that automatically create large structures. It is like watching Lego bricks that build a house or car themselves! Currently I am doing a PhD research in this field. I need to program nanoparticles in just the right way, and analyze the structures they produce.

‘With a physics degree you can work in many different places besides the academic world. For example, you could work with devices like an MRI scanner in a hospital. Or work at a company as technician or manager to make sure the organization runs smoothly. Or would you rather help building energy efficient cars that also run fast? The choice is yours!’

In the meantime, LION’s Prof. Carlo Beenakker has worked hard on building a digital database of all theses until 1975 that were successfully written in Leiden to obtain a PhD in Physics. This contains a rich collection of dissertations in beautiful old fonts, theses in Latin, works in nostalgic old Dutch and masterpieces of famous physicists. Hugo de Groot wrote the first physics thesis in 1597, titled ‘De infinito, loco, et vacuo’. Cor Gorter obtained his PhD in 1932 with research on ‘Paramagnetische Eigenschaften von Salzen’. We also find Jan Beenakker —Carlo Beenakker’s father—back in the list. In 1954, he wrote his thesis on ‘De invloed van het heliumisotoop met massa 3 op de eigenschappen van vloeibaar helium II’ (The Influence of the Helium Isotope with Mass 3 on the Properties of Liquid Helium II). Son Carlo obtained his PhD in Leiden as well, but after 1975 so you won’t find it in the database yet. And how about this dissertation for historical heritage: Hendrik Lorentz’s thesis titled ‘Over de theorie der terugkaatsing en breking van het licht’ (On the Theory of Reflection and Refraction of Light). Lorentz later went on to win the Physics Nobel Prize in 1902.

Einstein’s Light, a new film about Albert Einstein, will be premiered on Monday 2 November during the Leiden International Film Festival (LIFF). Leiden University has contributed to the film as part of the University’s 440-year anniversary celebrations. Several Leiden professors More info

are also featured in the film.

Innovative spirits

Einstein’s Light by film maker Nickolas Barris and Leiden University is an ode to the scientific curiosity and the innovative spirit of Nobel Prize winners Albert Einstein and Hendrik Lorentz. Around a century ago, Einstein was professor by special appointment at Leiden University.

The foundation of the theory of relativity was laid in Leiden, due in part to the construction of the Old Observatory in 1860 and the subsequent scientific discoveries. ‘Lorentz and Einstein met one another frequently at the University and they worked together on a regular basis. They were totally different personalities, but they admired one another,’ commented Jan van Ruitenbeek, Leiden Professor of Experimental Physics.

Light

Barris uses film, based on light, to illustrate the power of science. The title of the film, Einstein’s Light, isn’t just about research on light. Barris explains: ‘It embodies the scientific exploration of light, but also gives ‘light’ or guidance to the modern pursuit of expanding knowledge.’ The influence of music on Einstein’s creativity is an important theme, a theme that is highlighted with a soundtrack composed specially for the film and performed by Grammy Award-winning violinist Joshua Bell and pianist Marija Stroke. Watch the trailer

Four centuries of freedom

The film Einstein’s Light will be preceded by another special premiere: Vier eeuwen vrijheid (Four centuries of freedom). This short film is based on the book Edele, wijze, lieve, bijzondere by Professor of University History Willem Otterspeer. The book was published this year on the occasion of the 440th anniversary of the foundation of Leiden University.

Premiere of Einstein's Light in the Trianon

A hundred years after the development of the theory of relativity, the world premiere of the film will take place on Monday 2 November during the Leiden International Film Festival (LIFF), in the Trianon cinema. The cinema will be open from 19:00 hrs., and the programme is expected to end at 22.30 hrs. There are still free tickets available! As is appropriate for a special occasion, the dresscode for the evening will be lounge suits.

present E. T. Jaynes' concept of entropy as an inference tool and clarify when and how entropy can be used in the study of complex networks. Entropy is a property which indicates the spread of a probability distribution and has two applications: it can be used to determine a prior distribution when information on constraints is available; and it can be used to infer constraints when the distribution is known. I will review and discuss applications of entropy to the study of the arrow of time, language, food webs and firm sizes, as well as to algorithms of image reconstruction and trade flow estimation and reconciliation. Some of these applications can be interpreted within Jaynes' framework while others cannot. To conclude, I will identify current theoretical and empirical challenges in the study of economic networks and suggest how entropy can help to overcome them. These problems stem from the arbitrary nature of classifications and the aggregation of official statistical trade data, which make the analysis scale-dependent. Entropy can be used to identify correlations among trade flows, trade-offs in the uncertainty of disaggregated data and estimate firm and transaction size distributions.

During the month September, LION organized its first Image Award. The organization is delighted with the many beautiful contributions. A big thanks to all who have worked hard on sending in their image! The judges had a though time deciding More info

between some amazing pieces of art. In the end, they unanimously selected a well-deserved top 3. Congratulations to all!

Winner

Maarten Leeuwenhoek

Still life of diffraction patterns created by KOH etched square holes in Gold on Silicon Nitride. Image taken by an optical microscope with black and white sensor. Kandinsky would be proud!

Runner up

Group Oosterkamp

Shown in this figure is a very small magnetic particle, 300 times smaller than a millimeter, glued on a very tiny needle, the cantilever. We use this magnetic particle on the cantilever in our nano-MRI microscope, where the magnet replaces the extremely big magnet in conventional hospital-MRI's. Working 273 degrees below Celcius, close to absolute zero, our goal is to measure the force of a single nuclear spin using this magnetic cantilever. Currently, we are working on making this magnetic particle even smaller.

Third Place

Merlijn van Deen

In this experiment, we blow a large number of small bubbles and pack them together using a glass plate on the top and four wipers on the sides. We use these wipers to deform the system from the edges, and we track how the bubbles in the middle of the system respond to this. This allows us to understand what happens on a microscopic scale in a wide range of systems, such as a pile of sand, shaving foam or emulsions such as mayonnaise.

The winner Maarten Leeuwenhoek will receive his prize during a short ceremony at the start of the Ehrenfest Colloquium on November 4th.

Biophysicists at the Leiden Institute of Physics, under supervision of Prof. Martina Huber, and the University of Wageningen have used a new technique to monitor how certain proteins rearrange into a functional structure. With a smart tracing technique (DEER), the More info

researchers studied the folding process of the protein flavodoxin. Inside a cell, each protein has to restructure itself by folding into a new arrangement. Errors in this process can cause several diseases, like cystic fibrosis and neurodegenerative diseases like Alzheimer’s. Tomorrow, the scientific publication will appear in print in the Journal of Physical Chemistry.

Proteins play an important role in biological processes that also happen in our body. Each type of protein consists of a unique series of amino acids, which can take on numerous structures. To execute certain tasks—like electron transmission in photosynthesis, in the case of flavodoxin—a protein needs to fold into a specific structure. This is as essential for its function as the particles that it consists of. But where it is easy to determine the contents, it is far from clear how the ‘folding’ of proteins works. For this, we need information about the structure during multiple stages in the folding process. By understanding this process, scientists are able to see what happens when something goes wrong and a physical defect arises. In the end, this enables development of medicine against illnesses caused by errors in the folding process.

Labels
The Leiden research group used an existing technique for distinguishing materials in a sample: Electron Paramagnetic Resonance (EPR). Apart from radicals and certain metal ions, biophysicists can trace the atomic group nitroxide with EPR, and smart biochemical methods make it possible to attach the nitroxide group to a preferred site on the protein molecule. Lead author Martin van Son and his colleagues realized they could glue the nitroxide onto the protein to use it as a label to study the folding process. Just like an actor jumps around in front of a camera to make an animation movie, the protein folds in front of the EPR detector with labels attached to it. Since the distance between labels is monitored at each stage of the process, biophysicists can reconstruct a movie. During the development phase of the method, they only placed one set of labels on the protein, resulting in a low resolution. In the future, Huber will work with more labels, so she can see in higher resolution which part of the protein starts folding first.

Snapshots
Using EPR, you cannot record the whole movie in one go. The trick is to pause the folding process and take a snapshot. Unfortunately, it is not an option to simply resume the process from there. This is why the research team prepares multiple samples with proteins that haven’t completely folded yet, but instead have stopped at a different stage in the process. This stage depends on the concentration of denaturant that the researchers add to each sample. Now by measuring each sample, they take several snapshots that together form a movie. From the sequence of intermediate structures that a protein takes on before it finally arrives at its final shape, Huber deduces what reactions take place within the protein that induce folding.

Image caption: Intermediate step in the folding process of flavodoxine.

Leiden physicists have developed a way to address how accurately a superconducting single photon detector (SSPD) can be characterized by detector tomography. SSPDs are not fully understood, and tomography is a key element to determine how these devices detect light. More info

A regular CCD camera captures billions of light particles—called photons—each second. The plentitude of photons creates a macroscopic view of the everyday world we live in and allows us to take pictures with an ordinary camera. But in some cases, you might want to capture just a few photons. For example in quantum communication, where you exchange photons one at a time to share a secret key. If an outsider has been eavesdropping, the intended recipient will know because quantum mechanics dictates that a measurement changes the information stored in the photon. In the act of eavesdropping, the outsider leaves a clear trace. And just image how much night vision goggles would benefit from being able to detect single infrared photons. Astronomers would be delighted to spot extremely dim galaxies, and doctors could trace singlet oxygen particles that break down cancer cells.

For all these applications we need a device that is designed to capture single photons at non-conventional wavelengths. An example of such a detector is an SSPD, similar to the one that first author Qiang Wang used in his research; a series of tiny niobium nitride wires—just a hundred nanometers (nm) wide and 4 nm thick—cooled down to a temperature of 3.2 Kelvin to ensure that the wire is superconducting. In this state, electric currents flow without electrical resistance. Absorption of a photon weakens the superconductivity and may induce a transition where the wire becomes resistive. The current through the wire leads to a measurable voltage that comprises the detection of single photons. Surprisingly however, not every absorbed photon triggers the detector. So in addition to not all photons being absorbed, there is also an inefficiency in the electronic detection process of photons that were already absorbed.

Given their huge potential, scientists are eager to improve and use SSPDs. ‘Before these detectors can reach their full potential, we need to understand better how they work,’ says De Dood. ‘For that, we need new methods to characterize them, and I think detector tomography is the best way. It enables us to distinguish absorption inefficiency from electronic detection inefficiency. Before drawing firm conclusions, we need to determine the accuracy and practical limitations of the new method.’ And now De Dood’s research team has found a way to do precisely that. It is one step ahead in the process of understanding the mechanisms behind SSPDs.

This Saturday, Leiden University organizes its bi-annual open day in the city centre. Participants can gather information about all the bachelor studies that Leiden has to offer. There is plenty of opportunity to ask all questions you might have to More info

After finding evidence for a possible emission line at 3.5 keV in the Andromeda galaxy and the Perseus cluster last year, an international team of scientists has found the same signal when looking at the center of the Milky Way. More info

This discovery provides strong support to connect this emission line with the decay of dark matter, possibly answering one of the most important questions in modern astrophysics. Lead author Alexey Boyarksy from the Leiden Institute of Physics and his colleagues publish their results today in Physical Review Letters as an Editor’s Choice article. It is already online in pre-print on ArXiv.

We have known for decades that there is a lot of mass missing in the Universe—as much as eighty percent of the Universe’s mass is a total mystery. We can’t see it, we can’t explain it, but we know it’s there, based of movements of galaxies. The enigmatic nature of this dark matter has grown to be one of the biggest mysteries in science. Leiden physicist Alexey Boyarsky and his fellow researchers might just have found what so many scientists are after. Last year, they spotted an X-ray signal in the Andromeda galaxy and the Perseus cluster at an energy of 3.53 keV with the XMM-Newton space telescope, which they couldn’t directly link to any known source of normal matter. Around the same time, a group at Harvard University saw the exact same signal in the combined spectra of 73 galaxy clusters. In both observations the signal is real with more than 99.3 percent certainty. The question remains: where does it come from?

In a new study, the researchers looked at XMM-Newton data taken from the center of the Milky Way. With 99.9987 percent certainty, they saw an emission line at 3.539 keV. In addition, the signal strength abides upper and lower limits that were imposed by the two previous studies. It is stronger than the signal from other galaxies, which are farther away. And because the physicists didn’t detect a signal in the Milky Way’s thin halo, the signal from the center shouldn’t be stronger than a certain level, otherwise the halo should also give off a detectable signal.

In their publication, the scientists conclude that although it is hard to completely exclude a normal matter origin, it is even harder to find a known substance that can account for the observations in the different astronomical objects. The 3.53 keV signal could indeed come from decaying dark matter. Boyarsky has booked more time on the XMM-Newton telescope to study X-rays from a dwarf galaxy that is known for having few chemical elements. If his group finds the same signal again, the suspect emission line has little alibi left. Boyarsky: ‘Of course it is not guaranteed that we find the signal in the dwarf galaxy. We did what we could, and now we can either get lucky or unlucky. If we do find it there as well, that would be very, very strong evidence of dark matter. Then many further observations will follow.’

The Leiden Science Faculty organizes a team run to support the Foundation for Refugee Students UAF. This charity helps educated refugees to continue their studies or find a job at their own level. In many cases, obtained degrees More info

in the country of origin are not acknowledged in The Netherlands. Therefore, starting a new studies is of vital importance for many refugees. UAF advices, mediates and provides financial support. Each refugee student gets a personal coach. The foundation needs 4,500 euro per refugee per year to execute their program.

The Teams Science Run starts this Saturday at 12:00 at the University Sport Centre (Einsteinweg 6). Please don’t forget to donate to your favorite team through this website! That also provides you with the full list of participating teams. Teams 6, 18, 35, 36 and 47 are physics teams. Take your pick and help refugees build a fulfilling life in The Netherlands!

This afternoon the winner will be announced of the 2015 Physics Nobel Prize. Who wins and based on what discovery? What does this mean for physics research? To what extend do Leiden physicists participate in that research area? Theoretical More info

Physics Professor Carlo Beenakker answers all these questions in a special colloquium. All students and employees of the science faculty are welcome.

On Saturday October 24th, the Leiden Institute of Physics organizes an alumni day for all former students and employees. The participants will hear about the latest discoveries and research developments and get back in touch with familiar faces. The schedule More info

is filled with lectures on cell dynamics, microspectroscopy, mechanical metamaterial behavior and applications of astronomy techniques for the food industry. In between, there is plenty of time for socializing during an extensive lunch and the closing reception. Provided that there are enough participants, shuttle buses will run from train station Leiden Central around 10am. At the bottom of this text you will find information on how to sign up.

One of the challenges of modern material science is to create artificial structures whose unconventional mechanical response can be programmed by suitable design of their geometry or topology. Such structures termed mechanical metamaterials can fold in controlled ways or direct sound propagation. In this talk, we discuss metamaterials that exploit topologically protected mechanical states, which that could be used as building blocks for molecular robotics or information storage and read-out. The topological design rules we adopt will be brought to your fingertips using live demonstrations with macroscopic prototypes based on origami-like and cellular structures.

Lecture Dr. Doris Heinrich: Guiding Cells Towards Regeneration

Our insight into cellular behaviour is advanced by profound understanding of cytoskeleton dynamics and cellular response to external cues. The very survival of many cell types in the human body depends upon their perpetual active motions, which require internal structural changes. Control of cell functions will find wide applications in novel diagnostic assays and in regenerative medicine.

Lecture Prof. Dr. Michel Orrit: Microspectroscopy of Single Molecules and Single Gold Nanoparticles

The optical isolation of single gold nanoparticles leads to their spectroscopic study on a single-particle basis and, through their plasmonic properties, to the study of their direct surroundings. This general idea will be illustrated with some recent experiments from our laboratory.

After studying physics and obtaining his PhD at Leiden University, Marco Beijersbergen founded a company for developing custom-made measurement systems—Cosine Measurement Systems. These systems have applications for astronomy and other science projects, but also for practical problems on earth.

Sign Up
You can sign up until October 15th by filling out the form at the bottom of this link. We ask you to transfer €10,- to Universiteit Leiden at NL78RABO0102468869 citing SAP2008502041, as confirmation of your participation.

Tonight at 19:30, Paul Townsend (Cambridge, DAMPT) will kick off this year's series of the famous Colloquium Ehrenfestii. These lectures are organized about eight times a year, in the main auditorium of the Oort building.

Twist Again - Revisiting the relation between supersymmetry, twistors and the division algebras

The Lorentz and conformal algebras in spacetime dimension D=3,4,6,10 are isomorphic to Sl(2;K) and Sp(4;K) for K=R,C,H,O, the four division algebras (the definitions are non-standard for K=O because octonionic multiplication is not associative). These are also the dimensions that allow a super-Yang-Mills theory, for which the free-field
limit can be found by quantization of the massless superparticle.

Many years ago it was found that the D=3,4,6 massless superparticle has a manifestly superconformal formulation in terms of supertwistors; i.e. spinors of the superconformal group. The mass-shell constraint is replaced by U(1;K) ``spin-shell’’ constraints that determine the supermultiplet helicities. More recently, a supertwistor formulation has been found for the massive superparticle, which can be viewed as a special case of the massless superparticle in D=4,6,10,11. The mass-shell constraint is now replaced by U(2;K) constraints which imply that the quantum superparticle describes a (massive) supermultiplet of zero "superspin".

An account will be given of this circle of ideas relating division algebras supersymmetry and twistors in the simple context of particle mechanics.

We are saddened by the news that our former colleague Henk Rijskamp passed away on September 9. For many years Henk was a valued member of our technical support staff, working both in the old Kamerlingh Onnes Laboratory, and later More info

on in the Huygens building. He worked for the "practicum", became responsible for safety, and in the last years of his working career was a member of the safety department of the Faculty of Mathematics and Natural Sciences. He retired in 2003, and put his energy in community gardening and supporting primary schools in Leiderdorp. The last years of his life were marked by his battle with esophageal cancer.

Abstract:
In 2014, as the Ebola epidemic in West Africa progressed, the world witnessed in all its horrific glory a complex network in action. Doctors, epidemiologists, complex network scientists, and data scientists have worked hard to counter the disease, and now More info

the outbreak appears to be almost over. From a complex networks perspective the Ebola case affords a possibility to see how our theories hold up, and what there is to be learned. In this talk I will give a brief overview of the Ebola outbreak from a data science perspective, and discuss some of the issues that occurred as science met the real world.

Abstract
In 2014, as the Ebola epidemic in West Africa progressed, the world witnessed in all its horrific glory a complex network in action. Doctors, epidemiologists, complex network scientists, and data scientists have worked hard to counter the disease, and now the outbreak appears to be almost over. From a complex networks perspective the Ebola case affords a possibility to see how our theories hold up, and what there is to be learned. In this talk I will give a brief overview of the Ebola outbreak from a data science perspective, and discuss some of the issues that occurred as science met the real world.

Preliminary LCN2 Seminar Schedule - 2015
March 27, 16:00-17:00, HL214
April 24, 16:00-17:00, HL214
May 29, 16:00-17:00, HL214
June 26, 16:00-17:00, HL214
September 25, 16:00-17:00, HL214
October 30, 16:00-17:00, HL106
November 27, 16:00-17:00, HL106
December 18, 16:00-17:00, HL214'>Ebola. Big Data and Complex Networks in the Real World'.
Abstract
In 2014, as the Ebola epidemic in West Africa progressed, the world witnessed in all its horrific glory a complex network in action. Doctors, epidemiologists, complex network scientists, and data scientists have worked hard to counter the disease, and now the outbreak appears to be almost over. From a complex networks perspective the Ebola case affords a possibility to see how our theories hold up, and what there is to be learned. In this talk I will give a brief overview of the Ebola outbreak from a data science perspective, and discuss some of the issues that occurred as science met the real world.

Abstract
In 2014, as the Ebola epidemic in West Africa progressed, the world witnessed in all its horrific glory a complex network in action. Doctors, epidemiologists, complex network scientists, and data scientists have worked hard to counter the disease, and now the outbreak appears to be almost over. From a complex networks perspective the Ebola case affords a possibility to see how our theories hold up, and what there is to be learned. In this talk I will give a brief overview of the Ebola outbreak from a data science perspective, and discuss some of the issues that occurred as science met the real world.

It is with great sadness that we announce the parting of
Prof. Dr. L.N.M. Duysens, emeritus at the Leiden Institute of Physics of Leiden University, on September 8, 2015, at the age of 94 years.
Lou Duysens received his More info

PhD in physics at the University of Utrecht in 1952. His thesis on "Transfer of excitation energy in photosynthesis" initiated a new research field. After some years in the United States as a postdoctoral researcher he was appointed as lector at the physics department of Leiden University on October 1, 1956, and as full professor on January 1, 1962. There he built a highly successful biophysics group studying the mechanism of photosynthesis and won world-wide recognition for his work in this field. He retired in 1986.

Researchers at the Leiden Institute of Physics have developed an exciting technique to visualize electrical conductance in sheet-like materials. It shows great promise for devices based on a new family of materials—the ‘Van der Waals materials’. The physicists, who won More info

The fascinating properties of graphene—a single layer of carbon atoms—have been widely celebrated. Not only does graphene exhibit remarkable physics, it also shows great promise for new applications, like flexible display screens and solar cells. But scientists aren’t easily satisfied. The next generation of graphene-like materials is already under development. The Leiden physicists place atomically thin layers on top of each other, like a layered cake or ‘spekkoek’. For each layer they use carbon (graphene) or other materials like boron nitride or tungsten disulfide. Since these layers interact by Van der Waals forces, we simply call them Van der Waals materials.

Designer materials
First authors Jaap Kautz and Johannes Jobst design their materials layer by layer, enabling them to programme new properties and explore new physics. There is a large interest in using such materials for charge transport research and electronic applications. For that, it is important to understand what determines (or limits) the current flow in Van der Waals materials. The typical experimental approach is to apply electrodes to a sample, send a current and measure the resistance. However, this yields no detail on where the voltages drop. In fact, small-scale variations can strongly affect the conductance properties of the whole sample. Therefore, the research team of Sense Jan van der Molen designed a novel imaging technique that visualizes where the voltage actually drops: low-energy electron potentiometry (LEEP).

Maps of the local potential
LEEP is based on LEEM (low-energy electron microscopy)—a microscopy technique that uses ‘slow’ electrons to probe a surface very accurately. The reflection probability of these electrons depends strongly on the exact energy with which they hit a surface. This is due to a quantum mechanical effect: if the incoming electron has the exact same energy as an empty electron state in the Van der Waals material, it will not be reflected, but will get absorbed instead.
In fact, the reflectivity turns out to be so sensitive that local potential (voltage) variations can be detected. This is because an incoming electron’s energy depends precisely on the potential at a particular spot. If the potential is negative, the incoming electrons are decelerated and land with lower energy. If the potential is positive, they are accelerated. So if you apply a voltage over a Van der Waals material via two electrodes—meaning that the electrical potential changes with distance—there should be a variation in the local reflectivity. By accurately measuring the reflectivity for each point on a sample, researchers can determine the local potential (voltage) on the nanoscale.

Novel possibilities
The Leiden team has now demonstrated this promising new technique on arguably the simplest Van der Waals material possible. Kautz and Jobst applied it to electrically contacted stacks of graphene layers (1–3 layers). This allowed them to study the difference in conductance properties between single and triple layer graphene. They expect an extension to other Van der Waals materials to be straightforward. LEEP will also allow research on dynamic effects, for example as a function of temperature, since it is much faster than other potentiometry techniques. That creates opportunities for studying metal-insulator transitions, edge channels in topological insulators and ballistic charge transport in graphene. The latter might form the basis of a whole new line of graphene electronics. Jobst recently received a Veni grant to explore this exciting new direction.
Jaap Kautz and Johannes Jobst were awarded the 2015 NeVac prize, from the Dutch Vacuum Society, for their research.

(a) & (b) With LEEM, the whole sample is at the same potential, which determines the electron energy and wavelength. (c) With LEEP, a voltage is applied with two electrodes, giving each spot a different local potential, depending on its distance to the left electrode.

Art and science combined: Johannes Jobst made the LEEP potential map into a cake!

Using LEEP, the Leiden physicists measure the local conductivity of a sample by looking how waves of changing contrast move through the field of view. This video stems from the published article in Nature’s Scientific Report.

Tomorrow, the LION exam committee welcomes a new, external member. As of 1 September, at least one of the exam committee’s members has to be from outside Leiden University. The LION committee is very pleased that Dr. Stephan Eijt from More info

TU Delft has accepted their invitation to take a seat as the fifth member. Eijt is already chairman of the exam (sub)committee BSc Applied Physics in Delft, and therefore brings much experience to the table, according to prof. Jan van Ruitenbeek, who chairs the LION exam committee. ‘Plus we have an intensive collaboration with Delft. This exchange helps with that.’

The exam committee is responsible for the overall level of exams, and meets five times a year to discuss individual deviations from the standard curriculum and the implementation of a binding study advice. Students are never excluded through an automated process. If someone fails to meet the requirements for a positive study advice, and thinks this is due to personal circumstances, the committee discusses this and might grant a pardon.

Leiden physicists have discovered that a free, magnetised plasma reconfigures into a stable shape in certain conditions. They demonstrate through computer simulations that initially helical magnetic field lines in a plasma eventually take the form of a donut. Their paper More info

The Sun holds a great promise for solving the global energy problem. Aside from developing efficient solar panels, scientists are trying hard to fully understand the processes that happen inside our host star. They are eager to learn about one of the sun’s special tricks – nuclear fusion. With a fusion reactor on Earth we would be able to generate nearly unlimited amounts of energy. But to start up an efficient fusion process, we have to peek very carefully, with special emphasis on plasma’s behaviour within the complex magnetic fields inside stars.

Mathematical description
Up until now, no description existed of the reconfiguration of helical magnetic field lines within a free plasma. Now scientists at the Leiden Institute of Physics (LION) have created a computer simulation which shows that plasma, starting from different helical configurations, end up in a toroidal shape. They take the parameters from this simulation to formulate a mathematical description of the field lines. The fact that a donut shape is the end product of different initial configurations, shows that this form is a fundamental structure of a free plasma.

Energy state
Another indication towards that conclusion is that a torus is not just the minimum energy state, into which structures usually degrade, but a delicate balance between the present magnetic forces and the internal pressure within the plasma. Normally it is assumed that a magnetic configuration degrades into a form without forces acting on it, like a weak pudding collapsing without a mould. The Leiden finding contradicts the so-called Taylor’s theorem.

Islands
In the peaceful state of equilibrium, magnetic islands appear on the torus’ surface. ‘One of the issues with nuclear fusion is the emergence of small islands in the plasma,’ says lead author Christopher Berg Smiet, PhD student of prof. Dirk Bouwmeester at LION. ‘They fly outwards and damage the reactor wall. In our simulation we see stable islands that don’t do this. We’re talking about a different situation, because our plasma is not captured inside a reactor, but it does give us more insight into plasma’s behaviour.’

This insight is also important for our fundamental understanding of astrophysical processes. Smiet: ‘There are countless of helical magnetic plasma structures in the Sun, which sometimes fly off into space. Now we know that these turn into donuts after a while. This is something we had never expected beforehand.’

The 2015 Dissertation Prize of the Global Neutrino Network has been awarded to three former graduate students, amongst others Tri Astraatmadja, who was part of the Leiden/Nikhef ANTARES group during his PhD research. Astraatmadja receives the award for his thesis More info

'Starlight between the waves: In search of TeV photon emission from Gamma-Ray Bursts with the ANTARES Neutrino Telescope'.

This is the first year that the GNN Dissertation Prize is awarded. It recognises young postdoctoral candidates who have written an outstanding thesis and contributed significantly to the project. Primary criteria of the selection are the scientific quality, the didactics and the form of the thesis.

Astraamadja has focused on the ANTARES telescope, which operated as a gamma-ray telescope. This is possible by searching for down-going muons produced in interactions of gamma-rays in the Earth’s atmosphere. He looked at the short time windows when satellite experiments had announced a Gamma Ray Burst (GRB). The sophisticated tools developed by Astraatmadja will make it possible for the much bigger KM3NeT detector to detect gamma-rays from a GRB with a significance of three standard deviations.

The Netherlands eScience Center has announced to fund a new Path-Finding Project led by Dr. Dorothea Samtleben from the Leiden Institute of Physics (LION). This project aims to make the processing of detection signals more efficient for the KM3NeT neutrino More info

telescope, which is currently under construction in the Mediterranean Sea. High processing efficiency is vital for finding lower-energy neutrinos and the ability to alert other observatories in case of a special astronomical event.

Neutrinos are almost massless particles that have extremely little interaction with anything, so they undisturbedly speed through the Universe, even permeating stars. So when they are produced inside a star, they smoothly travel outwards and eventually reach Earth with all information still intact about their creation. Also, neutrinos have zero charge, so they are not deflected by any magnetic field and point directly to their origin. This makes these tiny particles a valuable source of information for astrophysicists, to study the spectacular events that produce them.

Detection
Unfortunately, their properties also make neutrinos incredibly hard to detect. Scientists need large volumes of material to hunt them down, like for example the water of the Mediterranean Sea where the KM3NeT neutrino telescope is under construction. Once in a while, one out of billions of neutrinos interacts with the water in the vicinity of the detector, and produces a light signal, which gets recorded. The trick is to process these signals and deduce the incoming particle’s characteristics as efficient as possible. When this happens fast enough, even real-time observation is possible. In that scenario the computers at KM3NeT can immediately alert their counterparts at optical observatories about a special astronomical event, so they’re able to photograph it. And they can extract online accurate information on the neutrino event candidates, to improve their selection efficiencies also for lower-energy neutrinos.

Collaboration
‘For real-time observation we need professional computing expertise to efficiently address the filtering of our huge data volume,’ says Samtleben. ‘The actual value of this grant is an eScience engineer to work with us for a full year. But mostly we’re establishing this first connection with the eScience center, to build a fruitful collaboration where their expertise and tools will enhance the science potential of the KM3NeT neutrino telescope. And they get a great playground to explore their tools and expertise. This could lead to a long-lasting partnership.’

Researchers at FOM-institute AMOLF and the Leiden Institute of Physics (LION) have developed a rubber rod with strange bending behaviours. Beyond a certain point, it bends more under decreasing pressure. This behaviour doesn’t fit our expectations and does not conform More info

to secular laws that predict the bending process. The rod is made out of metamaterial – material with special properties that are not found in nature. By providing the rod with a carefully selected pattern of small holes, the researchers managed to induce the strange behaviour. They published their work, a collaboration with Harvard University, in Physical Review Letters on July 21.

The metamaterial makes up a rubber rod of about twenty centimeter containing a pattern of elliptically shaped holes. These holes give the metarod its special property. The physicists noticed that a self-amplifying effect occurs at a turning point under a certain pressure level: the rod keeps bending further, while the pressure decreases. Group leader Martin van Hecke: ‘Imagine pushing a car forward. You expect to make the car go faster by pushing harder, but in this case the car speeds up if you push less.’

Custom-made bending
The researchers started looking for a mechanism that could explain this counter-intuitive effect. They discovered that you can easily squeeze together the metamaterial with a slight push, but it strongly resists stretching when you pull. Regular materials without holes show no significant difference between pushing and pulling, unless extremely high pressures are at play. This sensitivity to the difference between pulling and pushing causes the strange effect during bending of the metamaterial rod. The shape and place of the holes precisely determine the moment this effect occurs. So by changing the hole pattern, the scientists can custom-make materials with specific properties.

Ancient building principles
The development of the metamaterial rod challenges century-old building principles. The relationship between bending and pressure was established 250 years ago by Leonhard Euler, in his universal law of elastic instability of rods. Since then, his theory has formed the basis for building houses and bridges. Euler assumed that materials only show a distinction between pushing and pulling under extreme pressure. The new metamaterial rod displays a different behaviour, opening doors for new developments. ‘Building bridges with our metamaterial rod is never going to happen,’ says van Hecke. ‘But I can imagine a robot arm that is able to bent in a clever way thanks to mechanical switches that are based on this new material.’

Johannes Jobst - researcher at the Leiden Institute of Physics (LION) – has been awarded a Veni grant from the Netherlands Organisation for Scientific Research (NWO). Veni grants are handed out to very talented researchers who have recently obtained a More info

PhD. Jobst receives the maximum amount of 250,000 euro and will use the money to study how electric switching affects free-flying electrons in graphene transistors.

Graphene is made up of just a single layer of carbon atoms, and has shown great promise for a wide range of applications since its discovery in 2004. The material even earned a Physics Nobel Prize in 2010, awarded to Kostya Novoselov and Dutchman Andre Geim. One of the amazing properties of graphene is its incredible conductivity; electrons travel large distances without changing direction. Compared to conventional semiconductors, that are commonly used in electric devices, this promises great performance improvements.

The key to improving computers is the computer chip – a series of many tiny electric switches, or transistors. Simply put: the faster transistors switch, the faster computers are. And more of them means a more powerful computer. We cannot make traditional silicon transistors any faster, and for the past years the main effort has been put into making them smaller in order to squeeze more on a chip. However, we’ll soon reach a minimum size beyond which this transistor faces limitations, putting technology progress to a halt.

The solution may lie in graphene transistors. They switch faster than silicon and therefore increase computers speeds. ‘In theory they are a thousand times faster,’ says Jobst. ‘So instead of three gigahertz computers, we'd have clock rates of three terahertz. And because electrons fly freely in graphene without scattering, they produce much less waste heat, meaning a lot less power consumption.’

But computers should also work in practise, not just in theory. What will happen when we actually use graphene in electric switches? How will the electric field, used to switch such transistors, affect the free-flying electrons? By using a novel technique, based on low-energy electron microscopy (LEEM), Jobst is going to find out what happens. The Veni grant enables him to perform his research at LION and Columbia University, New York for three more years as senior postdoctoral researcher.

The novel microscopy technique that Jobst will use in his Veni research allows him to image where electrons scatter in graphene. In this example, the transition from single layer to triplelayer graphene (marked by arrows) can be identified as a major source of scattering.

Today, the French Optics Society (SFO) has announced the winners of its biennial Grand Prix SFO Léon Brillouin. Professor Michel Orrit from the Leiden Institute of Physics (LION) is one of two people that receive the honour this year. The More info

prize was established to honor the memory of physicist Léon Brillouin (1889-1969), whose various works have profoundly influenced the development of optics. The Grand Prix is awarded every odd year, after a jury has selected the winners. This week is the second in a row in which Orrit achieves big success. Last week, he and his research group were awarded a FOM grant.

A team of Dutch and Swiss researchers has managed to successfully scan the surface of a superconducting light detector in very high detail. Amongst them is Jelmer Renema, researcher at the Leiden Institute of Physics (LION). The new technique they More info

used could dramatically change the design of light detectors. These devices have many applications, amongst others in quantum information technology.

Superconducting light detectors are well equipped to measure individual photons. For this reason, these devices have become very popular in science and technical applications – for example communication with space probes and monitoring cancer therapies.

‘The problem with superconducting light detectors is, however, that they cannot observe every single entering photon,’ says Renema. ‘Up until now the reason behind that was unclear. We have established the cause of this limitation; certain parts of the detector are less sensitive to light than others. When a photon hits one of these areas, chances are that it remains undetected.’

Within the light detector is a flat nano wire - about 5 nanometer thick and 100 nanometer wide - which is folded very precisely, such that it forms a tightly packed, winding pattern. When a photon hits the wire, the detector immediately notices a small voltage peak. The researchers discovered that the middle of the wire has more trouble detecting light than the edges. For successful detection, a small whirlpool of electric current needs to form at the edge of the wire. When the photon directly hits this area, the process is more efficient.

The research team reached this conclusion by scanning the detector’s surface with a specially designed exposure technique. They targeted the detector with light of two different polarisations. Afterwards, they subtracted one measurement from the other. In this way, they created a higher contrast image than usual, which enabled the researchers to observe details that were previously invisible.
The research was conducted by scientists from FOM, Leiden University (LION), TU Eindhoven and Zurich University, as part of the FOM programme ‘Nanoscale Quantum Optics’.

Superconducting light detectors play an important role in quantum communication - an encrypted communication technique that involves photons in a secret quantum state. This communication is more efficient with superconducting light detectors. Bank transactions and other discrete messages are fundamentally safe with quantum communication. Also quantum computers could use this type of communication. These computers work in a principally different way than regular computers and are much faster, creating a whole range of revolutionary applications.

Today, two out of the five awarded FOM Project Space subsidies have been granted to the Leiden Institute of Physics (LION). The Foundation for Fundamental Research on Matter (FOM) divides a total of 2.2 million euro amongst the five winning More info

research groups. Prof. Jan Zaanen and prof. Koenraad Schalm receive a grant for research on ‘The strange metals: when quantum entanglement reaches its extreme’. Prof. Michel Orrit was granted money for his proposal called ‘An optical GPS for single electrons’. With the subsidy—around 500,000 euro—he is able to hire a PhD student for the full four years, plus a post-doc researcher for one year, and make some investments.

‘The main goal is to optically observe a single charge in a metallic island, which is often a golden nano particle,’ says Orrit. He will charge this nano particle through electron tunneling, and measure this with a single molecule. Orrit was thrilled to hear about the news. ‘I only saw it a few hours ago when I opened my email. I’m very happy with this grant and it couldn’t have come at a better time. With a new PhD student we have a perfect overlap. I was afraid of losing a lot of knowledge in my group because one of my researchers is leaving soon.’

Zaanen and Schalm receive the equivalent of five post-doc years to execute theoretical research on highly collective forms of entanglement in strange metals. These strange metals are famous for their very peculiar physical properties that cannot be explained with established many-particle quantum physics. Resting on mathematical methods developed in string theory (holographic duality) the two Leiden professors will explore the hypothesis that strange metals are maximally entangled states of quantum matter—a type of matter different from the everyday matter that we experience in daily life. Zaanen: ‘This research is the most exciting thing today in ultramodern physics. We might revolutionise our understanding of the world in the way Einstein did back in the 1910’s.’

The summerschool, from July 5 to July 15, is host to 24 students from
9 different countries. There will be 34 lectures by 19 LION lecturers, plus laboratory visits, showing the full breath of Leiden physics.
More info

Characterizing the behaviour of strongly coupled quantum systems out of equilibrium is a cardinal challenge for both theory and experiment. With diverse applications ranging from the dynamics of the quark–gluon plasma to transport in novel states of quantum matter, establishing More info

universal results and organizing principles out of equilibrium is crucial. We present a universal description of energy transport between quantum critical heat baths in arbitrary dimension.

In this year's round Milan Allan has been successful in obtaining a NWO Vidi grant. Out of the 509 proposals, the NWO committee decided to grant his proposal “A new perspective on the mysterious quantum soup”. Vidi is targeted at More info

the excellent researcher who has demonstrated the ability to independently generate and effect innovative ideas. We congratulate Milan with this success.

Jacob Bakermans (UL) presentation was titled "A novel way to control STM-based manipulation: motion tracking and rapid molecular dynamics simulation. The other contestants were Freek Roelofs (RU)and Luuk Vermont (UU). The jury was impressed by all 3 studies but ultimately More info

chose Jacob Bakermans as the winner. He received flowers and the prize of 500 euro from SPIN-chairman Peter Bouwmeester and NNV chairman Jan Ruitenbeek

Disordered packings of soft particles and disordered collections of coupled springs are models for materials as diverse as foams, emulsions, sand and (bio)polymer networks. For all these, the network of contacts or connections governs their mechanics, and both classes of More info

materials lose rigidity when the connectivity becomes too low. By comparing networks that are at the verge of failure, we uncover that packings exhibit a very strong response to the addition or removal of only one bond: every connection is crucial. Adding one bond leads to a rigid state where all bonds carry a load, and removing one bond leads to he complete fragmentation leading to many tiny patches connected by freely moving hinges. In random networks such dramatic response is completely absent, and even novel types of networks constructed to mimic packings fail to capture the unusual level of structural self-organization in sphere packings.
These insights can contribute to improved designs of network materials: if every element optimally participates in bearing loads, one can maximize e.g. the strength-to-weight ratio.

In amyloid fibrils, proteins assemble into nanometer-sized columns that have the flexibility of steel. The glue that converts the spaghetti-like proteins to assemble into self-repeating structures that form the fibril are normal inter-protein interaction forces, so the magic lies in More info

the precise arrangement of the protein.
We installed nano-sized markers, measured distances and found - based on gross approximations - a possible arrangement of the fibril for Parkinson's protein alpha-synuclein.
This peek into amyloid structure comes from the Electron Paramagnetic Resonance group of Martina Huber in Leiden in collaboration with the Twente/AMOLF group of Subramaniam.
Read more in Applied Magnetic Resonance

The Casimir Research School yearly awards the Hendrik Casimir Prize to outstanding MSc students. Usually, there are two winners per year. This year the jury had selected three equally outstanding students. The Casimir Director decided to award all three with More info

the Hendrik Casimir Prize. This year’s award winners are Anne Meeussen (Leiden), Jorinde van de Vis (Leiden), and Sander Konijnenberg (Delft). They each will receive a certificate and a sum of €750,- . The prize is based on the revenues from a donation to the Leiden University Fund by the late Josina Casimir-Jonker, widow of the famous Hendrik Casimir.